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/merge-pr.md

@@ -0,0 +1,28 @@
+# Template: merge a PR (eligible reviewer only)
+
+Copy, fill the `<...>` fields, and paste as the task prompt.
+
+```text
+Task: merge PR #<pr> for issue #<n> if it is eligible and checks pass.
+
+Rules (llm-project-workflow):
+- Only an eligible, NON-author reviewer merges. If authenticated user == PR
+  author → STOP.
+- Do not merge unless the PR is open, mergeable, and its checks/review pass.
+- No force-merge, no bypassing branch protections.
+
+Steps:
+1. Verify authenticated identity + active profile.
+2. Confirm PR #<pr>: author (not you), state open, mergeable, review approved.
+3. If any gate fails → STOP and report.
+4. Merge with explicit confirmation (e.g. confirmation="MERGE PR <pr>"),
+   optionally pinning the reviewed head SHA / changed-file set.
+5. Confirm remote master now contains the merge commit.
+
+Then run the cleanup template (worktree-cleanup.md):
+- close/release issue #<n>, remove status:in-progress
+- delete remote branch, remove local branch + worktree folder
+- fetch/prune; confirm main checkout is clean and current (0 0).
+
+Handoff: reviewer identity, merge result + commit, cleanup done, issue closed.
+```