FEAT: add Tombi as default TOML formatter

[redeboer]

• Closes Switch to Tombi as TOML formatter and linter #570
• Closes Merge developer configurations into pyproject.toml #98

✨ New features

• Add Tombi as the default TOML formatter and linter, replacing Taplo and toml-sort. The check-dev-files format command now installs the tombi-format and tombi-lint pre-commit hooks, writes a [tool.tombi] configuration to pyproject.toml, and recommends the tombi-toml.tombi VS Code extension.
• Add a --toml-formatter option (taplo or tombi, default tombi) so a repository can opt back into the previous Taplo + toml-sort setup.
• Generate a JSON Schema (compwa-policy.schema.json) for the public [tool.compwa.policy] configuration table and associate it through Tombi's [[tool.tombi.schemas]], so the policy config is validated and documented in the editor. The schema is regenerated by self-check.
• Embed the Settings field docstrings into the generated schema (via use_attribute_docstrings), so each policy option carries an inline description.
• Add a --tombi-errors-on-warnings option (default on) that runs tombi-lint with --error-on-warnings.

⚙️ Enhancements

• Run check-dev-files (and self-check) before the formatter hooks in the sorted .pre-commit-config.yaml, so generated files are formatted in the same pass.
• Split the Poe upgrade task into upgrade-precommit, upgrade-uv, and upgrade-pixi helper tasks referenced from the parallel task.
• Define CLI option types through typing.Literal instead of Enum.
• Drop the more-itertools dependency and use itertools.pairwise (Python 3.10 is the minimum version).
• Standardize TOML array formatting through to_toml_array() and stop enforcing single-line arrays.

⚠️ Interface changes

The Literal type aliases for policy options were consolidated into compwa_policy.config:

Old name	New name
compwa_policy.github.upgrade_lock.Frequency	compwa_policy.config.UpgradeFrequency
compwa_policy.env.conda.PackageManagerChoice	compwa_policy.config.PackageManagerChoice
compwa_policy.python.ty.TypeChecker	compwa_policy.config.TypeChecker

❗ Behavioral changes

• New repositories are configured with Tombi instead of Taplo + toml-sort by default; running the policy migrates existing repositories (removing taplo/toml-sort hooks, .taplo.toml, and [tool.tomlsort]).
• tombi-lint treats warnings as errors by default.
• Lock files (uv.lock) and other manifest files (Cargo.toml, Manifest.toml, Project.toml, labels*.toml) are excluded from Tombi formatting.
• The Ruff flake8-builtins setting is written under the renamed ignorelist key instead of builtins-ignorelist.

Squash commit messages

* BEHAVIOR: treat Tombi warnings as errors by default
* BREAK: remove `Frequency` type alias
* DOC: embed docstrings in JSON schema
* ENH: define options through `Literal` instead of `Enum`
* ENH: do not enforce non-multiline array
* ENH: exclude lock files from being formatted with Tombi
* ENH: run `check-dev-files` before formatters
* ENH: standardize formatting with `to_toml_array()`
* FEAT: generate JSON schema for `[tool.compwa.policy]`
* FEAT: implement `--toml-formatter` argument
* FEAT: implement `tombi-lint` pre-commit hook
* FEAT: implement warnings as errors for Tombi
* FIX: remove outdated Sphinx target remappings
* FIX: run `tombi-format` on GitHub CI
* MAINT: remove `more_itertools`
* MAINT: remove `PackageManagerChoice` from `conda` module
* MAINT: rename `builtins-ignorelist` to `ignorelist`