feat: diagnose reviewer lease handoff next actions (#599)

Add read-only diagnose_reviewer_pr_lease_handoff and MCP tool
gitea_diagnose_reviewer_pr_lease_handoff so open-PR foreign leases,
instructed-lease replacement, reclaimable release, and worktree
binding mismatch return a canonical next_action without steal paths.

Closes #599

gitea_mcp_server.py

@@ -7154,6 +7154,62 @@ def gitea_assess_reviewer_pr_lease(
     }
 
 
+@mcp.tool()
+def gitea_diagnose_reviewer_pr_lease_handoff(
+    pr_number: int,
+    proposed_worktree: str | None = None,
+    instructed_session_id: str | None = None,
+    instructed_comment_id: int | None = None,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Read-only: diagnose open-PR reviewer lease handoff and emit next action (#599).
+
+    Classifies no-lease / own / foreign / instructed-lease-missing-with-replacement
+    and worktree binding mismatch. Returns a canonical ``next_action`` without
+    mutating Gitea state. Never steals foreign leases.
+    """
+    read_block = _profile_operation_gate("gitea.read")
+    if read_block:
+        return {
+            "success": False,
+            "reasons": read_block,
+            "permission_report": _permission_block_report("gitea.read"),
+        }
+    comments = _fetch_pr_comments(
+        pr_number, remote=remote, host=host, org=org, repo=repo)
+    h, _o, _r = _resolve(remote, host, org, repo)
+    try:
+        username = _authenticated_username(h)
+    except Exception:
+        username = None
+    session_lease = reviewer_pr_lease.get_session_lease() or {}
+    env_wt = (
+        (os.environ.get("GITEA_REVIEWER_WORKTREE") or "").strip()
+        or (os.environ.get("GITEA_ACTIVE_WORKTREE") or "").strip()
+        or None
+    )
+    # Prefer in-session lease id when present; otherwise diagnose as a fresh
+    # caller without inventing ownership of an on-thread lease.
+    session_id = (session_lease.get("session_id") or "").strip() or None
+    diagnosis = reviewer_pr_lease.diagnose_reviewer_pr_lease_handoff(
+        comments,
+        pr_number=pr_number,
+        current_session_id=session_id,
+        current_reviewer_identity=username,
+        proposed_worktree=proposed_worktree,
+        env_bound_worktree=env_wt,
+        instructed_session_id=instructed_session_id,
+        instructed_comment_id=instructed_comment_id,
+    )
+    diagnosis["success"] = True
+    diagnosis["remote"] = remote
+    diagnosis["authenticated_user"] = username
+    return diagnosis
+
+
 @mcp.tool()
 def gitea_cleanup_post_merge_moot_lease(
     pr_number: int,