feat: add scripts/release-tag automation helper (#50)

Automate the documented release-tag checklist (#48) without bypassing safety
gates.

scripts/release-tag:
- Requires a SemVer tag (vMAJOR.MINOR.PATCH); validates before any git/network.
- Fetch/prune first, then refuses: dirty worktree, non-master branch, local
  master != remote master, HEAD not on remote master, and an existing local or
  remote tag of the same name.
- Runs the full suite by default; --skip-tests is an explicit opt-out that warns.
- Creates an ANNOTATED tag (git tag -a), never lightweight.
- Safe by default: no push unless --push; --dry-run prints planned actions and
  changes nothing. Supports --notes-file <path> for the annotation message.
- Prints: commit, tag, tests_run, tag_created, tag_pushed.
- Env injection points for testing/CI: RELEASE_TAG_REMOTE, RELEASE_TAG_TEST_CMD.

tests/test_release_tag.py (14 cases): valid SemVer dry-run; invalid version;
dirty worktree; non-master; master/remote mismatch; existing tag; missing
notes-file; annotated-not-lightweight; no-push-without-flag; push-only-with-flag;
notes-file message; --skip-tests warns; default runs tests (fail blocks tag,
pass tags). Each test builds a throwaway repo with a LOCAL bare remote (cloned,
not pushed) and stubs the test command — no network, no real tags, no pushing
from the project repo.

Docs: reference scripts/release-tag from the runbook, SKILL, and the release-tag
template (script preferred; manual steps are the fallback).

Full suite 305 passed / 0 failures; bash -n clean; git diff --check clean; no
secrets.

Closes #50. Refs #48.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/llm-workflow-runbooks.md

@@ -368,6 +368,18 @@ Release runbook (see [`../skills/llm-project-workflow/templates/release-tag.md`]
 6. `git tag -a <vX.Y.Z> prgs/master -m "<notes referencing #issues / PRs>"`.
 7. `git push prgs <vX.Y.Z>`; add release notes if the forge supports it.
 
+`scripts/release-tag` automates steps 1–7 with these gates built in (SemVer
+check, fetch/prune, on-master, clean tree, local==remote master, HEAD on remote
+master, no duplicate tag, tests run unless `--skip-tests`, annotated tag only).
+It is **safe by default** — no push unless `--push`, and `--dry-run` changes
+nothing:
+
+```bash
+scripts/release-tag --dry-run v0.4.0
+scripts/release-tag v0.4.0 --notes-file /tmp/release-notes.md
+scripts/release-tag v0.4.0 --notes-file /tmp/release-notes.md --push
+```
+
 ## Safety notes
 
 - Never place raw tokens or passwords in any LLM MCP config; reference secrets