fix(changelog): add changelog_subject_only to skip body parsing

[bearomorphism]

Description

Closes #1267.

Why

commitizen.changelog.generate_tree_from_commits() parses both the commit subject and each \n\n-separated block of commit.body against commit_parser. So a commit such as

feat: new feature

refactor: incidental cleanup

produces two changelog entries:

### Feat
- new feature
### Refactor
- incidental cleanup

This is rarely what the author intended — they wrote one feat, the body was just a note. The maintainer ack on #1267 confirms the body-parsing is undesirable, but flipping the default would be a behaviour break for existing users (some intentionally rely on it for multi-message commits).

What changed

File	Change
commitizen/defaults.py	New TypedDict field changelog_subject_only: bool and DEFAULT_SETTINGS["changelog_subject_only"] = False.
commitizen/changelog.py	generate_tree_from_commits() gains a subject_only: bool = False kwarg. When True, the body iteration is replaced with an empty tuple, so only the subject is matched.
commitizen/commands/changelog.py	Changelog.__call__ forwards self.config.settings["changelog_subject_only"] to generate_tree_from_commits(). This single call site is shared by cz changelog and cz bump --changelog.
tests/test_changelog.py	New test_generate_tree_from_commits_subject_only_skips_body_blocks exercises both modes against a commit whose body contains a parser-matching block.
tests/commands/test_changelog_command.py	New test_changelog_subject_only_setting_skips_body_parsing is an end-to-end test (real config file, real cz changelog invocation) that catches typos in the config-key wiring.
tests/test_conf.py	Two expected-Settings literals updated to include the new key (otherwise the strict-equality assertions in TestReadCfg::test_load_conf fail).
docs/commands/changelog.md	New ### changelog_subject_only section next to the existing --merge-prerelease documentation.

How it works

1. Default-off semantics. changelog_subject_only = false is the default in both the Settings TypedDict and DEFAULT_SETTINGS, so BaseConfig.__init__ always seeds the dict-key with False and the dict-index access in Changelog.__call__ cannot KeyError.
2. Single chokepoint. Body iteration happens at one place — the chain([subject_match], (body_map_pat.match(block) for block in commit.body.split("\n\n"))) in generate_tree_from_commits(). Replacing the second argument of chain with () when subject_only=True is the entire behavioural change.
3. Why from __future__ import annotations matters. The new body_matches: Iterable[re.Match[str] | None] annotation only resolves in TYPE_CHECKING blocks at runtime; without from __future__ import annotations the import would have to leave TYPE_CHECKING. The file already had it.
4. The end-to-end test was specifically requested. Both reviewers (sonnet-4.6 and GitHub Copilot) flagged that the lib-level test alone wouldn't catch a typo in the self.config.settings["changelog_subject_only"] lookup at commands/changelog.py. The fixup commit 292feb4f adds an integration test that constructs a real config and runs cz changelog --dry-run, then verified by hand that the test fails (with KeyError) when the config key is mistyped.

Backward compatibility

• The default false preserves the historical body-parsing behaviour for every user who hasn't opted in.
• All 158 pre-existing tests/test_changelog.py and tests/commands/test_changelog_command.py tests pass unchanged.
• The new subject_only kwarg on generate_tree_from_commits() defaults to False, so any external caller that imports the function continues to work.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude following the guidelines

Code Changes

• Add test cases to all the changes you introduce (1 unit test + 1 integration test)
• Run uv run poe all locally to ensure this change passes linter check and tests (poe lint clean; 159 changelog tests pass)
• Manually test the changes (see "Steps to Test" below)
• Update the documentation for the changes

Documentation Changes

• Added a ### changelog_subject_only section to docs/commands/changelog.md
• Run uv run poe doc locally to ensure the documentation pages render correctly
• Check and fix any broken links (internal or external)

Expected Behavior

Setting	Subject feat: X + body refactor: Y	Result
changelog_subject_only unset (default false)	Both subject and body block parsed	Two entries: one feat, one refactor (legacy)
changelog_subject_only = true	Only subject parsed	One entry: feat: X

Applies to both cz changelog and cz bump --changelog.

Steps to Test This Pull Request

git fetch fork fix/1267-changelog-subject-only
git checkout fork/fix/1267-changelog-subject-only

# 1. Lib-level regression test (forces the body-skip behaviour at the call site).
uv run pytest tests/test_changelog.py -k subject_only -v

# 2. End-to-end test (real config file, real cz changelog invocation).
uv run pytest tests/commands/test_changelog_command.py::test_changelog_subject_only_setting_skips_body_parsing -v

# 3. Verify the e2e test would catch a typo in the config-key wiring.
sed -i.bak 's/changelog_subject_only/changelog_subject_only_TYPO/' commitizen/commands/changelog.py
uv run pytest tests/commands/test_changelog_command.py::test_changelog_subject_only_setting_skips_body_parsing -v
# expected: KeyError: 'changelog_subject_only_TYPO'
mv commitizen/commands/changelog.py.bak commitizen/commands/changelog.py
uv run pytest tests/commands/test_changelog_command.py::test_changelog_subject_only_setting_skips_body_parsing -v
# expected: 1 passed

# 4. Full changelog suite (no regressions).
uv run pytest tests/test_changelog.py tests/commands/test_changelog_command.py -q
# expected: 159 passed

Additional Context

Surfaced while triaging open issues in #1965. Maintainer ack on #1267 confirmed body-parsing is at-best opt-in. The end-to-end integration test was added in fixup commit 292feb4f after both review passes (sonnet-4.6 and Copilot) flagged the wiring as the only path not exercised by tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (4b93a50) to head (292feb4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1974   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files          61       61           
  Lines        2779     2782    +3     
=======================================
+ Hits         2730     2733    +3     
  Misses         49       49           

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📢 Have feedback on the report? Share it here.

[Copilot]

Pull request overview

Adds a new configuration setting to control whether changelog generation should parse commit body blocks as additional changelog entries, addressing issue #1267 while preserving historical behavior by default.

Changes:

• Introduces changelog_subject_only (default false) to skip parsing \n\n-separated commit body blocks when generating changelogs.
• Plumbs the setting through the Changelog command so cz changelog (and cz bump --changelog via the same command) can honor it.
• Adds a regression test for generate_tree_from_commits(..., subject_only=True) and documents the new setting.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
commitizen/changelog.py	Adds subject_only parameter to skip body-block parsing when building the changelog tree.
commitizen/commands/changelog.py	Passes changelog_subject_only from config into changelog tree generation.
commitizen/defaults.py	Adds changelog_subject_only to typed settings and default settings.
tests/test_changelog.py	Adds regression coverage for subject-only parsing behavior.
tests/test_conf.py	Updates expected/default settings fixtures to include changelog_subject_only.
docs/commands/changelog.md	Documents the new changelog_subject_only setting and its effect.

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[bearomorphism]

Closing this PR per maintainer-triage policy: feature-request issues should sit with the maintainers for design / scope review before any implementation lands. The issue's type: feature (or implicit equivalent) label means it's not on a "ready-to-implement" track yet.

The implementation itself is preserved on the branch (fix/1267-changelog-subject-only) — if a maintainer decides this is the direction they want, the PR can be re-opened in one click, or the work can serve as a starting point for a maintainer-led design.

This PR is being closed so that #1267 reverts to "awaiting maintainer triage / decision" rather than "PR pending review", which is the correct state for a feature request.

Closed via the round-2 triage cleanup in #1965.