fix(mcp): silence the daemon-attach log by default (#618) (#725)

The "Attached to shared daemon" line is benign INFO, but it was written to
stderr — and MCP hosts render all server stderr at error level (and append an
`undefined` data field), so on every session start a healthy attach showed up
as `[error] … undefined`. It is now gated behind CODEGRAPH_MCP_LOG_ATTACH=1:
silent by default, opt-in for debugging daemon attach. Both attach sites
(runProxy + connectWithHello) route through one helper. The daemon integration
tests opt the harness into the log so their attach assertions still observe a
successful attach.

Re-applies the approach from #640 by @mturac.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

__tests__/daemon-attach-log.test.ts

@@ -0,0 +1,38 @@
+/**
+ * #618 — the "attached to shared daemon" line is benign INFO, but MCP hosts
+ * render server stderr at error level (and tack on an `undefined` data field),
+ * so on every session start a healthy attach showed up as `[error] … undefined`.
+ * It's now gated behind CODEGRAPH_MCP_LOG_ATTACH=1 — silent by default, opt-in
+ * for debugging. Approach from #640 by @mturac.
+ */
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { logAttachedDaemon } from '../src/mcp/proxy';
+
+const hello = { pid: 4242, codegraph: '9.9.9' } as any;
+
+describe('daemon attach log gating (#618)', () => {
+  let spy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    spy = vi.spyOn(process.stderr, 'write').mockImplementation((() => true) as any);
+  });
+
+  afterEach(() => {
+    spy.mockRestore();
+    delete process.env.CODEGRAPH_MCP_LOG_ATTACH;
+  });
+
+  it('is silent by default (no [error]/undefined noise in MCP hosts)', () => {
+    delete process.env.CODEGRAPH_MCP_LOG_ATTACH;
+    logAttachedDaemon('/tmp/cg.sock', hello);
+    expect(spy).not.toHaveBeenCalled();
+  });
+
+  it('logs the attach line only when CODEGRAPH_MCP_LOG_ATTACH=1 (opt-in debug)', () => {
+    process.env.CODEGRAPH_MCP_LOG_ATTACH = '1';
+    logAttachedDaemon('/tmp/cg.sock', hello);
+    const out = spy.mock.calls.map((c) => String(c[0])).join('');
+    expect(out).toContain('Attached to shared daemon on /tmp/cg.sock');
+    expect(out).toContain('pid 4242');
+  });
+});

__tests__/mcp-daemon.test.ts

@@ -52,7 +52,10 @@ function spawnServer(cwd: string, env: NodeJS.ProcessEnv = {}): SpawnedServer {
   const child = spawn(process.execPath, [BIN, 'serve', '--mcp'], {
     cwd,
     stdio: ['pipe', 'pipe', 'pipe'],
-    env: { ...process.env, ...env },
+    // #618: the daemon-attach log line is now off by default; opt the test
+    // harness into it (CODEGRAPH_MCP_LOG_ATTACH=1) so the attach assertions
+    // below can still observe a successful attach. A per-test env still wins.
+    env: { CODEGRAPH_MCP_LOG_ATTACH: '1', ...process.env, ...env },
   }) as ChildProcessWithoutNullStreams;
   // Swallow spawn/EPIPE errors so killing a child mid-write can't surface as an
   // unhandled error that crashes the vitest worker.