feat(workflows): add reusable release-prepare workflow

[ss-o]

Summary

• Adds .github/workflows/release-prepare.yml, a reusable (workflow_call) workflow for class-2 repositories that prepares a release on push to the default branch without publishing one: it computes the next semantic version from Conventional Commits since the last vX.Y.Z tag (feat → minor, fix/perf → patch, !/BREAKING CHANGE → major; only chore/docs/ci → clean no-op), drafts a changelog with GitHub Models via actions/ai-inference (grouped commit list as fallback when inference is unavailable), and opens or updates a single release-proposal issue with the draft notes and the exact annotated-tag commands.
• Documents caller usage in runbooks/release.md (new "Release preparation automation (class 2)" section with a caller snippet).
• The maintainer-pushed annotated tag remains the only publication act and the per-repo tag-driven release.yml (zunit pattern) still publishes, so ADR 0007 is unchanged. Callers own concurrency (per fix(trunk): let callers own concurrency #436); the reusable workflow does not set it.

Closes #444 tasks 1–2; pilots in zunit/zsh-lint are follow-ups in those repos.

Verification

• trunk check (actionlint 1.7.12, shellcheck, markdownlint, prettier): no issues on both changed files.
• Action SHAs verified against official tags via the GitHub API: actions/checkout@de0fac2e… = v6.0.2 (same pin as zsh-n.yml), actions/ai-inference@a7805884… = v2.1.1.
• Version-computation and changelog-grouping scripts dry-run verbatim against real z-shell/zunit history (v0.6.3 → proposes v0.7.0 minor; correctly ignores the non-conforming v0.7.0-alpha pre-release tag) and against a synthetic repo for the no-releasable-commits no-op, the !-breaking major bump, and the existing-tag guard.

Note: local commit/push hooks are currently broken repo-wide by the stale sh-hook trunk action (#445); this PR was pushed with --no-verify after trunk check passed manually.

Agent handoff

• Status: Ready for review
• Current state: Reusable workflow + runbook docs complete and validated.
• Blockers: None.
• Next steps: After merge, add thin caller workflows in z-shell/zunit and z-shell/zsh-lint (see Add reusable release-prepare workflow (semver proposal + AI changelog) for class-2 repos #444 task list; relates to zsh-lint#21).
• Related tracker/issues: Add reusable release-prepare workflow (semver proposal + AI changelog) for class-2 repos #444, Stale sh-hook trunk action blocks every local commit (.github/hook.sh removed) #445

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds a reusable GitHub Actions workflow to prepare (but not publish) releases for class-2 repositories, plus documentation showing how to wire it in as a caller. It aligns with ADR-0007 by keeping the maintainer-pushed annotated tag as the publication boundary while automating version proposal + draft notes.

Changes:

• Added a reusable workflow_call workflow that computes the next semver from Conventional Commits since the last vX.Y.Z tag, drafts release notes via GitHub Models (with a fallback), and opens/updates a single “release proposal” issue.
• Documented how class-2 repos should call the reusable workflow (including required permissions and concurrency guidance).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
runbooks/release.md	Documents the new class-2 “release preparation automation” and provides a caller workflow snippet.
.github/workflows/release-prepare.yml	Implements the reusable release-prep workflow (semver proposal, AI/fallback changelog drafting, issue creation/update).