feat(grounding-mcp): add ledger_status no-arg health verb

[LanNguyenSi]

Adds a ledger_status MCP tool to grounding-mcp so harness's MCP doctor can probe it with an empty-args tools/call.

Why

Every existing grounding-mcp tool requires a sessionId (or other args). Calling any of them with {} returns a JSON-RPC validation error, which harness's MCP probe treats as unhealthy. The example manifest in harness therefore cannot register grounding-mcp with a working health verb today (harness task bd4ee1c9).

What

• packages/grounding-mcp/src/ledger-bridge.ts: new ledgerStatus() helper. Read-only queries (SELECT COUNT(*), SELECT MAX(created_at)) wrapped in try/catch, returns a discriminated union { status: 'ok', dbPath, entryCount, lastWriteAt } | { status: 'error', message }.
• packages/grounding-mcp/src/server.ts: register ledger_status tool with no input schema, delegates to ledgerStatus().
• packages/grounding-mcp/tests/roundtrip.test.ts: two new Vitest cases.
  • Happy path: two ledger entries, asserts status=ok, entryCount=2, dbPath echoes the env var, lastWriteAt is a string.
  • Error path: writes non-sqlite bytes to a file, points EVIDENCE_LEDGER_DB at it, asserts status=error with a non-empty message. (First attempt used a missing parent dir but evidence-ledger's getDb auto-mkdirs, so the corrupt-file approach was needed.)

Dogfood

$ node dist/server.js  # with isolated EVIDENCE_LEDGER_DB
> tools/call { name: 'ledger_status', arguments: {} }
< { status: 'ok', dbPath: '/tmp/.../l.db', entryCount: 0, lastWriteAt: null }

Confirms harness's probe contract (tools/call with empty arguments) is satisfied.

Out of scope

• Other new tools (per task acceptance).
• Ledger schema changes.
• Auth / rate limiting.
• The pre-existing TypeScript deprecation warnings on server.tool(name, description, schema, cb) — every existing tool emits them; not introduced by this PR.

Evidence

.agent-grounding/evidence/feat/grounding-mcp-ledger-status.jsonl committed for the review-claim-gate auto-detect path.

Closes agent-tasks task 453d86f4.