fix(mcp): recover clean unpublished author work after the owning session exits

Closes #772

Recovery now dispatches on observed publication state: published_owning_pr
(remote branch exists; head equality #753 or strict descendancy #768,
unchanged) and unpublished_claim (no remote branch, no PR; ownership proven
by the durable lock record plus a local HEAD strictly descending from the
server-observed base the branch was cut from).

The absence of a remote head is never itself permission. Recovery writes are
compare-and-swap against a lock generation, and the mutating lock path and
read-only diagnostic assessor share one evaluator.

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

issue_lock_worktree.py

@@ -145,6 +145,83 @@ def read_head_ancestry(
     return result
 
 
+def read_recorded_base(
+    worktree_path: str,
+    *,
+    head_sha: str | None,
+    extra_bases: tuple[str, ...] | list[str] = (),
+    base_branches: frozenset[str] | None = None,
+) -> dict:
+    """Observe the base commit an unpublished claim was branched from (#772).
+
+    A published claim records its base implicitly: the remote branch head is the
+    thing recovery measures against. An unpublished claim has no remote ref, so
+    the base must be observed here, server-side, as the merge-base between the
+    worktree HEAD and the base branch it was cut from.
+
+    Reports facts only; the disposition lives in ``issue_lock_recovery``. Every
+    field is read from git in the declared worktree — nothing is supplied by, or
+    reachable from, an MCP caller, so a caller cannot nominate a base that would
+    make unrelated history look like a descendant (#772 AC1/AC4).
+
+    A HEAD with no common ancestor in any base branch yields ``probe_ok`` with no
+    ``base_sha``: unrelated history is reported as exactly that, never as a base.
+    """
+    path = (worktree_path or "").strip()
+    head = (head_sha or "").strip()
+    bases = base_branches or BASE_BRANCHES
+    candidates = [*extra_bases, *sorted(bases)]
+    result: dict = {
+        "base_branch": None,
+        "base_sha": None,
+        "head_sha": head or None,
+        "probe_ok": False,
+        "candidates": candidates,
+        "reasons": [],
+    }
+    if not path or not head:
+        result["reasons"].append(
+            "recorded-base probe requires a worktree path and a HEAD sha"
+        )
+        return result
+
+    probed_any = False
+    for candidate in candidates:
+        name = (candidate or "").strip()
+        if not name:
+            continue
+        probe = subprocess.run(
+            ["git", "-C", path, "merge-base", name, head],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if probe.returncode not in (0, 1):
+            # 0 = merge base found, 1 = no common ancestor. Anything else is a
+            # failed probe (missing ref, broken repo) — try the next candidate.
+            continue
+        probed_any = True
+        merge_base = (probe.stdout or "").strip()
+        if probe.returncode == 0 and merge_base:
+            result["base_branch"] = name
+            result["base_sha"] = merge_base
+            result["probe_ok"] = True
+            return result
+
+    result["probe_ok"] = probed_any
+    if probed_any:
+        result["reasons"].append(
+            f"HEAD {head} shares no common ancestor with any of "
+            f"{_base_list(bases)}; history is unrelated to this repository's base"
+        )
+    else:
+        result["reasons"].append(
+            f"recorded-base probe could not run against any of {_base_list(bases)} "
+            f"in '{path}'"
+        )
+    return result
+
+
 def read_worktree_git_state(
     worktree_path: str,
     extra_bases: tuple[str, ...] | list[str] = (),