docs: add Work Selection Rule for LLM operating rules

Require lease verification (open PRs, branches, worktrees, leases,
merged completion) before any issue or PR work. Surface the rule in
the portable skill, Gitea runbooks, operator guide, and start-issue
template; add doc-contract tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/templates/start-issue.md

@@ -13,6 +13,15 @@ Rules (llm-project-workflow):
 - Do not self-review or self-merge.
 
 Steps:
+0. Work Selection Rule — before any claim, branch, or file edits, acquire or
+   verify a work lease. Required checks: list open PRs; search PRs linked to
+   the target issue; search local/remote branches for the issue number; search
+   registered worktrees for the issue branch; check dirty worktrees; check
+   active leases or recent handoffs; check whether a merged PR already
+   completed the issue. If another session owns the lease, stop (continue only
+   as lease owner, review the existing PR, hand off, request takeover after
+   expiry, or report "work already claimed"). Never open a parallel branch/PR
+   unless the old branch is proven abandoned and takeover is recorded.
 1. Identity Checklist: Before claiming work, verify and state:
    - Required identity/profile for this task: author (allowed to push branches / create PRs)
    - Current authenticated identity (from whoami): <username>