feat(mcp): FastMCP server exposing read-only KB tools (stdio)

Implements F-14: thin FastMCP wrapper over existing codex domain modules.
Seven read-only tools: search, ask, wiki_read, wiki_list, discover_leads,
provenance_verify, synthesis_browse. All optional-feature tools degrade
gracefully to {"error": "feature not available"} instead of crashing.
Adds mcp[cli]>=1.0 dependency and codex-mcp console_script entry-point.
28 new tests across test_server, test_search, test_readonly, test_graceful,
test_http_auth — all green; 0 regressions in existing 172 tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -19,10 +19,12 @@ dependencies = [
     "tenacity>=8",
     "pymupdf>=1.24",
     "pix2tex>=0.1.4",
+    "mcp[cli]>=1.0",
 ]
 
 [project.scripts]
 codex = "codex.cli:app"
+codex-mcp = "codex.mcp_server:main"
 
 [dependency-groups]
 dev = [