fix(mcp): real cited bibkeys in wiki_read + constant-time token compare

- C-3 (MED): wiki_read returned 'sources' = [concept_slug] (a placeholder), not
  the cited sources. Now parses the page's inline [BibKey #loc] citations and
  returns the actual set of cited bibkeys.
- S-2 (LOW): _TokenAuth compared the Bearer header with != (length/equality
  timing side-channel). Use secrets.compare_digest for constant-time comparison.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

codex/mcp_server.py

@@ -24,6 +24,7 @@ Transport
 from __future__ import annotations
 
 import logging
+import secrets
 
 from mcp.server.fastmcp import FastMCP
 from mcp.server.transport_security import TransportSecuritySettings
@@ -121,7 +122,6 @@ def wiki_read(concept_slug: str) -> dict[str, object]:
     directory or the requested page does not exist.
     """
     try:
-        import json
         from pathlib import Path
 
         from codex.config import get_settings
@@ -135,16 +135,12 @@ def wiki_read(concept_slug: str) -> dict[str, object]:
 
         markdown = page_path.read_text(encoding="utf-8")
 
-        # Collect cited bibkeys from the compile state
-        state_path = wiki_dir / ".compile-state.json"
-        sources: list[str] = []
-        if state_path.exists():
-            try:
-                state: dict[str, str] = json.loads(state_path.read_text(encoding="utf-8"))
-                if concept_slug in state:
-                    sources = [concept_slug]
-            except (json.JSONDecodeError, OSError):
-                pass
+        # Collect the actual cited bibkeys from the page's inline [BibKey #loc]
+        # citations (audit C-3: this previously returned the concept slug itself,
+        # not the cited sources).
+        from codex.wiki import _parse_claims
+
+        sources = sorted({claim.bibkey for claim in _parse_claims(markdown)})
 
         return {"markdown": markdown, "sources": sources}
     except Exception as exc:  # noqa: BLE001
@@ -337,7 +333,9 @@ def main() -> None:
                 self, request: Request, call_next: RequestResponseEndpoint
             ) -> Response:
                 auth = request.headers.get("Authorization", "")
-                if auth != f"Bearer {token}":
+                # Constant-time comparison to avoid a token-length/equality timing
+                # side-channel (audit S-2).
+                if not secrets.compare_digest(auth, f"Bearer {token}"):
                     return Response("Unauthorized", status_code=401)
                 return await call_next(request)