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>

skills/llm-project-workflow/SKILL.md

@@ -250,3 +250,9 @@ Release process (see [`templates/release-tag.md`](templates/release-tag.md)):
 6. Create the annotated tag on remote `master` with release notes.
 7. Push the tag.
 8. Create/update release notes if the forge supports it.
+
+Where present, `scripts/release-tag` automates this with all gates built in
+(SemVer, fetch/prune, on-master, clean tree, local==remote master, HEAD on
+remote master, no duplicate tag, tests, annotated-only). Safe by default: no
+push without `--push`; `--dry-run` changes nothing; `--skip-tests` must be
+explicit and warns.

skills/llm-project-workflow/templates/release-tag.md

@@ -3,6 +3,18 @@
 Copy, fill the `<...>` fields, and paste as the task prompt. Tagging is
 irreversible-ish and outward-facing — fail closed on any doubt.
 
+> If the project ships `scripts/release-tag`, prefer it — it enforces every gate
+> below automatically and is safe by default (no push without `--push`,
+> `--dry-run` changes nothing):
+>
+> ```bash
+> scripts/release-tag --dry-run <vX.Y.Z>
+> scripts/release-tag <vX.Y.Z> --notes-file <path>
+> scripts/release-tag <vX.Y.Z> --notes-file <path> --push
+> ```
+>
+> The manual steps below are the fallback / what the script does.
+
 ```text
 Task: cut release <vX.Y.Z> from master.