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/review-pr.md

@@ -0,0 +1,26 @@
+# Template: review a PR
+
+Copy, fill the `<...>` fields, and paste as the task prompt.
+
+```text
+Task: review PR #<pr> for issue #<n>.
+
+Rules (llm-project-workflow):
+- Review in a SEPARATE detached review worktree, never the author's folder.
+- You must NOT be the PR author. If the authenticated user == PR author, stop.
+- Do not merge if any check fails.
+
+Steps:
+1. Verify your authenticated identity (whoami) and the active profile.
+2. Fetch the PR facts: PR author, head SHA, state (must be open), base branch.
+3. If authenticated user == PR author → STOP (no self-review).
+4. scripts/worktree-review <pr-head-branch>          # detached, branches/review-*
+   cd branches/review-<pr-head-branch-slug>
+5. Confirm the worktree is clean. Inspect the FULL diff; confirm scope matches
+   issue #<n>; flag any unrelated files, secrets, or formatting churn.
+6. Run the test suite; note results.
+7. Post the review verdict: approve only if scope is clean and checks pass;
+   otherwise request changes with specifics. Never merge from this review step.
+
+Handoff: reviewer identity, PR author, scope verdict, checks + results, decision.
+```