fix(reviewer-workflow): address review — in-process mutation authority, no /tmp lock (#199)

Addresses the sysadmin REQUEST_CHANGES on PR #203 (reviewed head
10d2644790):

1. Lock redesigned; /tmp file removed entirely. The mutation authority is
   now an in-process record (_MUTATION_AUTHORITY) plus an environment
   session lock (GITEA_SESSION_PROFILE_LOCK) exported at server launch:
   - in-process record cannot be spoofed by other local processes, cannot
     go stale across sessions, and cannot race concurrent agents;
   - the env lock is inherited by child CLI processes, so review_pr.py can
     refuse an ad-hoc GITEA_MCP_PROFILE role escalation without any shared
     file; a missing env lock (direct operator CLI use) stays allowed;
   - silent except-pass writes are gone; an unresolvable profile fails
     closed.
2. Standard reviewer workflow unbroken: verify_mutation_authority seeds
   itself from the live config-resolved context at the first mutation gate
   (approved preflight path whoami -> eligibility -> review/merge), and now
   runs as the final gate after eligibility, reusing the identity that
   eligibility proved (no extra /user call).
3. Trailing whitespace removed from review_pr.py (git diff --check clean).
4. Module-global verify_mutation_authority no-op bypass removed from
   tests/test_mcp_server.py; replaced with a tests/conftest.py autouse
   fixture that only resets per-process state (_MUTATION_AUTHORITY,
   _IDENTITY_CACHE, session lock env) between tests — the gate itself
   stays live in every test.
5. Tests rewritten for the new design: seeding on first verify, unresolved
   profile fails closed, remote/profile/identity mismatches fail closed,
   session-lock env mismatch rejected, foreign-pid authority reseeded,
   unauthorized author->reviewer pivot blocked, authorized pivot allowed;
   CLI: mismatch blocked, match allowed, no-lock allowed.
6. Rebased onto current master (c6fd0fd).

Closes #199
Refs #194

Co-Authored-By: Claude Fable 5 <[email protected]>

review_pr.py

@@ -60,28 +60,31 @@ def main(argv=None):
 
     host, org, repo = resolve_remote(args)
 
-    # ── Mutation Authority context wall check (Issue #194) ──
-    LOCK_FILE = "/tmp/gitea_mutation_authority.lock"
-    
-    if os.path.exists(LOCK_FILE):
+    # ── Reviewer mutation side-channel wall (#199, refs #194) ──
+    # The launching MCP session exports GITEA_SESSION_PROFILE_LOCK with the
+    # profile it was started with; child processes inherit it. If this CLI
+    # resolves a different profile — e.g. an ad-hoc GITEA_MCP_PROFILE
+    # override escalating an author-bound session to reviewer — refuse
+    # before any API call. No lock in the environment means no session
+    # context (direct operator CLI use), which stays allowed. Unlike a /tmp
+    # lock file, the environment is per-process-tree: other sessions cannot
+    # spoof it and it cannot go stale across sessions.
+    session_lock = (os.environ.get("GITEA_SESSION_PROFILE_LOCK") or "").strip()
+    if session_lock:
         try:
-            with open(LOCK_FILE, "r", encoding="utf-8") as f:
-                lock_data = json.load(f)
-            
-            # Resolve current CLI profile
-            cli_profile = get_profile().get("profile_name")
-            locked_profile = lock_data.get("current_profile")
-            
-            if cli_profile != locked_profile:
-                print(
-                    f"Mismatched active profile vs mutation profile (CLI override rejected): "
-                    f"CLI profile '{cli_profile}' does not match locked active profile '{locked_profile}' (fail closed)",
-                    file=sys.stderr
-                )
-                return 3
+            cli_profile = (get_profile().get("profile_name") or "").strip()
         except Exception as e:
             print(f"Mutation authority check failed: {e}", file=sys.stderr)
             return 3
+        if cli_profile != session_lock:
+            print(
+                f"Mismatched active profile vs session profile lock "
+                f"(CLI override rejected): CLI profile '{cli_profile}' does "
+                f"not match locked session profile '{session_lock}' "
+                f"(fail closed)",
+                file=sys.stderr,
+            )
+            return 3
 
     body = args.body
     if args.body_file: