fix(mcp): accept strict-descendant dead-session recovery (Closes #768)

Permit fail-closed recovery when a clean local head is a strict
descendant of the recorded PR/remote head, with server-side ancestry
proof. Propagate recovery evidence through commit, push, and PR
duplicate gates so an owning PR is not re-blocked as competing work.

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

issue_work_duplicate_gate.py

@@ -122,18 +122,30 @@ def _assess_owning_pr_exemption(
             f"the recovered branch '{token_branch}' (no owning-PR exemption)"
         )
         return False, notes
-    if not token_head or not only_sha or only_sha != token_head:
+    # #768: a descendant recovery is measured against the head the PR still
+    # shows, then publishes the local descendant — so the live PR head is the
+    # recorded head before that push and the accepted head after it. Both are
+    # server-derived and name the same owned PR, so both are accepted; anything
+    # else still fails closed.
+    token_accepted = str(recovered_owning_pr.get("accepted_head") or "").strip()
+    acceptable_heads = [head for head in (token_head, token_accepted) if head]
+    if not acceptable_heads or not only_sha or only_sha not in acceptable_heads:
         notes.append(
             f"open PR #{only_number} head {only_sha or 'unknown'} does not "
-            f"match the recovered head {token_head or 'unknown'} "
-            "(no owning-PR exemption)"
+            f"match the recovered head {token_head or 'unknown'}"
+            + (
+                f" or the accepted head {token_accepted}"
+                if token_accepted and token_accepted != token_head
+                else ""
+            )
+            + " (no owning-PR exemption)"
         )
         return False, notes
 
     return True, [
         f"open PR #{only_number} is the exact PR already owned by the "
         f"recovering lock for issue #{issue_number} (branch '{token_branch}', "
-        f"head {token_head}); not duplicate work"
+        f"head {only_sha}); not duplicate work"
     ]