Add release-cel2sql4j skill

[richardwooding]

Summary

Adds a project-local skill that captures the cel2sql4j release workflow — tag-driven, no CHANGELOG maintenance (release notes are auto-generated from PR titles), publishes to Maven Central via the vanniktech plugin.

Skill content

SKILL.md covers:

• Picking a semver bump (patch / minor / major) — with cel2sql4j's pre-1.0 caveat that breaking changes can land in a minor while still in 0.x.
• Common slip-ups: lightweight tag vs annotated, stale gradle.properties (intentional — release.yml overrides via ORG_GRADLE_PROJECT_VERSION_NAME), pre-release qualifier shape (v1.0.0-rc1), forgetting to merge security Dependabot PRs first.
• The four required org-level secrets (Maven Central token + GPG key; GPG must be RSA, not Ed25519).
• Post-tag verification — release URL, Maven Central sync delay (~30 min), what to do if the workflow fails mid-publish.

Preflight script

scripts/release_preflight.sh [<version>]:

• ✅ Validates branch (must be main), working-tree clean, in sync with origin/main (hard errors).
• ✅ Lists commits since the previous tag — the release-notes preview.
• ✅ Checks CI status on the current origin/main HEAD via gh. Only the CI workflow is release-blocking; the Automatic Dependency Submission (Gradle) workflow flakes on transient GitHub 5xx and is reported as (advisory).
• ⚠️ Lists open Dependabot PRs as a soft warning (security fixes should ideally land before the tag).
• ✅ If a version is supplied, validates the format (vX.Y.Z or vX.Y.Z-qualifier) and prints the exact git tag -a / git push commands.

Smoke-tested behaviour

Running ./release_preflight.sh v0.2.0 from this branch correctly reports:

ERROR: current branch is 'feat/release-skill', expected 'main'
ERROR: working tree has uncommitted changes
=== Commits since v0.1.0 ===
  5e8eaa6 Add three repo-local agent skills + fix missed Spark writeFormat impl (#11)
  ...
=== CI on origin/main HEAD ===
  failure      Automatic Dependency Submission (Gradle)  (advisory)
  success      CI
=== Version check ===
  OK:    version 'v0.2.0' format valid

That is, the script flags the real preconditions while correctly not blocking on the known submit-gradle flake.

Lint

SKILL.md is 104 lines (under the 200-line target) and passes python3 .claude/skills/skill-authoring/scripts/lint_skill.py cleanly.

Test plan

• Skill lints clean
• Preflight script runs end-to-end and reports correctly
• Build still green (./gradlew build)
• CI green on this branch

🤖 Generated with Claude Code