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_lock_worktree.py

@@ -30,6 +30,108 @@ def resolve_author_worktree_path(
     return os.path.realpath(os.path.abspath(path))
 
 
+def read_head_ancestry(
+    worktree_path: str,
+    *,
+    ancestor_sha: str | None,
+    descendant_sha: str | None,
+) -> dict:
+    """Observe whether ``descendant_sha`` strictly descends from ``ancestor_sha`` (#768).
+
+    Server-side git observation for dead-session lock recovery. The recovering
+    author's only reachable clean-worktree state is one commit *ahead* of the
+    head recorded at lock time, so recovery needs to know whether that commit
+    extends the recorded head or replaces it.
+
+    Reports facts only; the disposition lives in ``issue_lock_recovery``. Every
+    field is read from git in the declared worktree — nothing here is supplied
+    by, or reachable from, an MCP caller (#768 AC6).
+
+    ``ancestor_present`` proves the recorded head is still reachable, which is
+    what separates an honest fast-forward from a rewritten or force-moved
+    history: a rewritten recorded head leaves the object graph and the probe
+    fails closed.
+    """
+    path = (worktree_path or "").strip()
+    ancestor = (ancestor_sha or "").strip()
+    descendant = (descendant_sha or "").strip()
+    result: dict = {
+        "ancestor_sha": ancestor or None,
+        "descendant_sha": descendant or None,
+        "probe_ok": False,
+        "ancestor_present": False,
+        "descendant_present": False,
+        "is_ancestor": False,
+        "is_strict_descendant": False,
+        "proof": None,
+        "reasons": [],
+    }
+    if not path or not ancestor or not descendant:
+        result["reasons"].append(
+            "ancestry probe requires a worktree path and both commit SHAs"
+        )
+        return result
+
+    def _present(sha: str) -> bool:
+        res = subprocess.run(
+            ["git", "-C", path, "rev-parse", "--verify", "--quiet", f"{sha}^{{commit}}"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        return res.returncode == 0
+
+    try:
+        result["ancestor_present"] = _present(ancestor)
+        result["descendant_present"] = _present(descendant)
+    except OSError as exc:  # git unavailable — fail closed, never assume
+        result["reasons"].append(f"ancestry probe could not run: {exc}")
+        return result
+
+    if not result["ancestor_present"]:
+        result["reasons"].append(
+            f"recorded head {ancestor} is not reachable in '{path}'; history may "
+            "have been rewritten or force-moved"
+        )
+    if not result["descendant_present"]:
+        result["reasons"].append(
+            f"local head {descendant} is not reachable in '{path}'"
+        )
+    if not (result["ancestor_present"] and result["descendant_present"]):
+        return result
+
+    probe = subprocess.run(
+        ["git", "-C", path, "merge-base", "--is-ancestor", ancestor, descendant],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    # 0 = is an ancestor, 1 = is not. Anything else is a failed probe, not a "no".
+    if probe.returncode not in (0, 1):
+        result["reasons"].append(
+            f"ancestry probe failed with exit {probe.returncode}; ancestry unproven"
+        )
+        return result
+
+    result["probe_ok"] = True
+    result["is_ancestor"] = probe.returncode == 0
+    result["is_strict_descendant"] = result["is_ancestor"] and ancestor != descendant
+    result["proof"] = (
+        f"git -C <worktree> merge-base --is-ancestor {ancestor} {descendant} "
+        f"-> exit {probe.returncode}"
+    )
+    if not result["is_ancestor"]:
+        result["reasons"].append(
+            f"local head {descendant} does not descend from recorded head {ancestor}"
+        )
+    elif not result["is_strict_descendant"]:
+        result["reasons"].append(
+            f"local head {descendant} equals the recorded head; no descendant "
+            "recovery is involved"
+        )
+    return result
+
+
 def read_worktree_git_state(
     worktree_path: str,
     extra_bases: tuple[str, ...] | list[str] = (),