Detect release-please merge commit to fire release job

[pthm]

Summary

Fixes the gate that prevented v0.8.2 from auto-publishing when PR #56 (the release-please release PR) was merged.

Why

We run `release-please-action` with `skip-github-release: true` so the release job can do the staged tag dance — push `melange/vX.Y.Z` first, bump root `go.mod`, then tag root at the bumped commit. This guarantees `vX.Y.Z` lives on a commit whose `go.mod` is internally consistent with `VERSION`.

But `release-please-action` has a hardcoded safety check: if it finds a merged release PR with no corresponding tag, it aborts with `There are untagged, merged release PRs outstanding` — and crucially, it does not set `release_created: true`. The release job's gate (`needs.release-please.outputs.release_created == 'true'`) never fires.

The dry-run never caught this because the dry-run was triggered by `workflow_dispatch`, which the gate also accepts via `github.event_name == 'workflow_dispatch'`. The release-please-output branch of the OR was never exercised in any test.

What this PR does

1. Adds a third gate signal: `startsWith(github.event.head_commit.message, 'chore(main): release ')`. The release-please commit message format is stable and documented; that's how the rest of the GitHub ecosystem detects release-please merges too.

2. Consolidates version extraction into a single "Determine version" step that handles all three trigger sources:

   • `workflow_dispatch`: `github.event.inputs.version`
   • release-please owns the tag: `needs.release-please.outputs.tag_name`
   • merged release-PR push: parsed from the commit message (`chore(main): release 0.8.2` → `v0.8.2`)

   The job-level `VERSION` env is dropped in favour of `echo VERSION=... >> $GITHUB_ENV` so the parsing logic can run in shell.

Verified locally

```
INPUT: chore(main): release 0.8.2
OUTPUT: v0.8.2

INPUT: chore(main): release 1.0.0-rc.1
OUTPUT: v1.0.0-rc.1

INPUT: feat: something else
OUTPUT: (empty — would fail with "Could not extract version")
```

The downstream regex (`^v[0-9]+.[0-9]+.[0-9]+(-[A-Za-z0-9.]+)?$`) is unchanged.

Test plan

• Once merged, the next `feat:` or `fix:` to land on `main` produces a release-please PR
• Merging that PR fires the release job automatically (no manual `workflow_dispatch` needed)
• Dry-runs via `workflow_dispatch` still work (preserves the existing path)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.35%. Comparing base (8d5271b) to head (3d24a2b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   83.35%   83.35%           
=======================================
  Files          79       79           
  Lines       10397    10397           
=======================================
  Hits         8666     8666           
  Misses       1396     1396           
  Partials      335      335           

Flag	Coverage Δ
integration	78.13% <ø> (ø)
unit	25.18% <ø> (ø)

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