feat(mcp): Phase 12 — narrow MCP server + ousast mcp

A zero-dependency MCP server over stdio (newline-delimited JSON-RPC 2.0) for
OpenCode/IDE integration, exposing only stable project-level operations.

- mcp.py: McpServer (pure handle() dispatch) + serve() stdio loop, stdlib-only.
  Ten tools: openultrasast.scan/status/findings/get_finding/evidence/artifacts/
  benchmark/explain/propose_patch/export_report. scan/benchmark run the bounded
  analysis pipeline; NO tool accepts a free-form command — no shell, Docker, or
  internal hunter tools are exposed. propose_patch degrades visibly (patch oracle
  is a later phase). initialize/tools/list/tools/call/ping handled; notifications
  get no response; errors surface as JSON-RPC errors instead of crashing the loop.
- cli: `ousast mcp` subcommand launches the stdio server (lazy import → keeps the
  mcp↔cli cycle one-directional).
- tests/test_mcp.py: tool listing is exactly the narrow surface, surface-safety
  (no shell/exec/command tool), scan→status→findings→explain→evidence flow,
  export_report, propose_patch degradation, and the stdio loop round-trip.
- README "MCP server" section; openrouter-sast-harness Phase 12 checked off.

Verified end-to-end over real stdio (initialize → tools/list → scan). 179 passed,
4 skipped; ruff + mypy clean; detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

README.md

@@ -167,6 +167,24 @@ Project skills that steer the harness from OpenCode (`.opencode/skills/`):
 | `openultrasast-triage` ✅ | false-positive elimination, ranking calibration, verifier adjudication |
 | `openultrasast-fix-audit` ✅ | OpenUltraCode fix lifecycle and adversarial fix review |
 
+### MCP server (narrow surface) ✅
+
+`ousast mcp` runs a zero-dependency MCP server over stdio (newline-delimited
+JSON-RPC) exposing only stable, project-level operations — **never** arbitrary
+shell, Docker, or internal hunter tools. Point an MCP client (OpenCode, Claude
+Desktop, an IDE) at it:
+
+```jsonc
+// e.g. an MCP client config entry
+{ "command": "uv", "args": ["run", "ousast", "mcp"] }
+```
+
+Tools: `openultrasast.scan`, `status`, `findings`, `get_finding`, `evidence`,
+`artifacts`, `benchmark`, `explain`, `propose_patch` (degrades visibly — the patch
+oracle is a later phase), and `export_report`. The `scan`/`benchmark` tools run the
+bounded analysis pipeline; no tool accepts a free-form command. A typical flow:
+`scan {path}` → returns a `run_dir` → `findings {run_dir}` → `explain {run_dir, finding_id}`.
+
 **Ultra workflows (OpenUltraCode discipline):** fixes are never a one-shot patch
 prompt. `openultrasast-fix-audit` runs the bounded lifecycle
 `intake → plan → minimal patch → adversarial review → reconcile → fresh