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]>

tests/test_review_proofs.py

@@ -1051,6 +1051,8 @@ class TestControllerHandoff(unittest.TestCase):
         self.assertIn("## Controller Handoff", skill)
         self.assertIn("assess_controller_handoff", skill)
         self.assertIn("issue #182", skill)
+        self.assertIn("## Work Selection Rule for LLMs", skill)
+        self.assertIn("work already claimed", skill)
 
     def test_handoff_rejects_none_workspace_mutations_when_local_edits_exist(self):
         # 1. Workspace mutations: none is rejected when local_edits is True