docs: add portable llm-project-workflow skill + templates (#46)

Extract the project's operating rules into a reusable, project-agnostic skill
so any repo can adopt the same safe LLM workflow.

- skills/llm-project-workflow/SKILL.md: issue-first; isolated branch worktrees
  (main checkout = orchestration only); distinct author/reviewer identities and
  profile safety (secrets by reference only; stop if authenticated user == PR
  author); branch naming; start/review/merge/cleanup workflows; fail-closed
  cases; recovery patterns; and an "Adapting to a project" table for the
  forge-specific names.
- templates/: copy/paste prompts for start-issue, review-pr, merge-pr,
  recover-bad-state, worktree-cleanup.
- Link the skill from README.md and docs/llm-workflow-runbooks.md (the runbook
  is framed as the Gitea-specific application of the portable skill).

Docs-only; no code, no secrets, safe placeholder examples only. No change to
MCP runtime, Gitea API, credential storage, or worktree helpers.

Checks: full suite 287 passed / 0 failures; git diff --check clean; secret scan
of skills/ clean.

Closes #46. Refs #38, #39.

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

skills/llm-project-workflow/templates/worktree-cleanup.md

@@ -0,0 +1,28 @@
+# Template: clean up after merge
+
+Copy, fill the `<...>` fields, and paste as the task prompt. Only run AFTER a
+real merge is confirmed on remote master.
+
+```text
+Task: clean up branch/worktree for PR #<pr> / issue #<n> after merge.
+
+Rules (llm-project-workflow):
+- Do NOT clean up until the merge is confirmed on <remote>/master.
+- Cleanup would-delete-unmerged-work → STOP. Never --force-remove a dirty tree.
+
+Steps:
+1. git fetch <remote> --prune
+2. Confirm <remote>/master contains the merge of PR #<pr>
+   (git log <remote>/master | grep the merge, or git branch -r --contains <sha>).
+   If not merged → STOP; run the recovery template instead.
+3. Close issue #<n> if not auto-closed; remove the status:in-progress label.
+4. scripts/worktree-clean --delete-branch <type>/issue-<n>-<slug>
+   (removes branches/<type>-issue-<n>-<slug>; refuses if dirty; git branch -d is
+   safe-delete only — fails on unmerged.)
+5. Delete the remote branch if the merge did not already remove it.
+6. From the main checkout: git fetch <remote> --prune; git checkout master;
+   git reset --hard <remote>/master ONLY if local master safely matches remote.
+7. Confirm main checkout clean and current (git status; 0 0 vs <remote>/master).
+
+Handoff: merge confirmed, issue closed, branch+worktree removed, checkout clean.
+```