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>

README.md

@@ -186,6 +186,10 @@ Notes:
   [`docs/llm-workflow-runbooks.md`](docs/llm-workflow-runbooks.md) for the
   task-scoped, profile-based runbooks (create/review/merge/close, thin
   launchers, migration, fail-closed rules).
+- For the **portable** version of this workflow (issue-first, isolated
+  worktrees, no self-review/merge, profile safety, cleanup, fail-closed) that
+  can be copied into any project, see the reusable skill
+  [`skills/llm-project-workflow/SKILL.md`](skills/llm-project-workflow/SKILL.md).
 - **Audit logging (#18):** mutating actions emit a durable, redacted JSON audit
   record — timestamp, action, result (`allowed`/`blocked`/`failed`/`succeeded`),
   profile name + audit label, authenticated username, target repo/issue/PR,