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__/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.