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_operator_guide.py

@@ -132,7 +132,8 @@ class TestControlPlaneGuide(GuideTestBase):
         rules = g["rules"]
         for key in ("hard_stops", "fail_closed", "head_sha_pinning",
                     "merge_confirmation", "redaction", "separation",
-                    "profile_switching", "identity_verification"):
+                    "profile_switching", "identity_verification",
+                    "work_selection"):
             self.assertIn(key, rules)
         self.assertIn("MERGE PR", json.dumps(rules["merge_confirmation"]))
         self.assertTrue(rules["hard_stops"])