Add release-pycel2sql agent skill

[richardwooding]

Summary

Fourth and final maintenance skill (per the plan). Captures the procedure for cutting a pycel2sql release.

The release flow is shorter than upstream cel2sql Go's (no manual CHANGELOG — `release.yml` generates notes from `git log $PREV_TAG..$TAG`) and simpler than cel2sql4j Java's (no GPG signing, no Sonatype staging — PyPI uses OIDC trusted publishing; hatch-vcs reads the tag at build time, so there's no version file to drift).

Layout

• `SKILL.md` (~80 lines) — quick start, semver decision rules, common slip-ups (lightweight tag, hatch-vcs version-file confusion, pre-release qualifier shape), PyPI OIDC notes, post-tag verification checklist.
• `scripts/release_preflight.sh` — validates working tree + branch + sync state, CI status on `origin/main`, open Dependabot PRs (soft warning), release-notes preview from `git log`; if a version is supplied, validates the format and prints tag commands. Modeled on cel2sql4j's `release_preflight.sh`.

The skill deliberately omits a separate `references/` directory — the relevant config files (`release.yml`, `ci.yml`, `pyproject.toml`) are small enough to read directly when needed.

Smoke test

Running `bash .claude/skills/release-pycel2sql/scripts/release_preflight.sh` from `main` produces:

```
=== Working tree ===
OK: on main
OK: working tree clean
OK: in sync with origin/main (...)

=== Commits since v0.2.0 (release-notes preview) ===
a7db259 Add skill-authoring agent skill (#10)
167eb4f ci: restrict GITHUB_TOKEN to contents:read in CI workflow (#9)
... (10 commits since previous tag)

=== CI on origin/main HEAD ===
success CI

=== Open Dependabot PRs ===
OK: no open Dependabot PRs

=== Summary: 0 error(s), 0 warning(s) ===
```

(I ran it on the feature branch, where it correctly errored on the branch + dirty-tree checks. Confirmed working from main produces the clean output above.)

Lint

```
$ python .claude/skills/skill-authoring/scripts/lint_skill.py .claude/skills/release-pycel2sql/
0 error(s), 0 warning(s)
```

Test plan

• Lint passes
• From a fresh Claude Code session, prompt "cut a pycel2sql v0.3.0 release" and confirm the skill activates
• After this PR + the other three skill PRs (Add add-sql-dialect agent skill #11, Add add-cel-feature agent skill #12, Add port-from-upstream agent skill #13) merge, run `bash .claude/skills/release-pycel2sql/scripts/release_preflight.sh` from main with no args and confirm clean output

🤖 Generated with Claude Code