Dispatch MCP requests concurrently per connection

handleConn processed requests serially, so a slow tool (e.g.
wait_for_pattern with a 300s timeout) monopolized the single per-agent
MCP connection and every queued call timed out behind it. Handle each
request in its own goroutine, serialize responses through a per-conn
write mutex (full response written atomically, partial writes handled),
copy the request line before handing it off (bufio reuses its buffer),
and wait on a WaitGroup before closing the conn so in-flight handlers
finish cleanly. Greeting stays sequential; notifications still get no
response.

Resolves the [MCP TIMEOUT] TODO item.

TODO.md

@@ -1,5 +1,4 @@
 - [ ] Codex idle detection seems to trigger too soon, see below [CODEX IDLE]
-- [ ] Issue with mcp timing out [MCP TIMEOUT]
 - [ ] When opening a codex sub agent, the message gets input to the field, but the message is never submitted.
   - This appears to be inconsistent. Sometimes it works, sometimes it doesn't. Might be because of popups on codex sub agents?
   - Question: when it fails, is a Codex startup popup visible (trust/workspace, auth/model selection, permissions), or is the normal composer focused?
@@ -60,13 +59,3 @@ Crunched for 1m 57s
  ✔ Set up worktree for issue 136 implementation                                                                                                                                                                             │
  ✔ Draft implementation plan                                                                                                                                                                                                │
   … +2 completed                                                                                                                                                                                                            │
-
-
-# [MCP TIMEOUT]
-⚙ patterm_send_input [key=enter, kind=key, process_id=p_a6726d, submit=false, tail_mode=stream, text=, wait_ms=1000]                                                                                                    │
-                                                                                                                                                                                                                        │
-⚙ patterm_wait_for_pattern [pattern=Findings|No findings|No issues|Residual risk, process_id=p_a6726d, scope=scrollback, timeout_seconds=300]                                                                           │
-MCP error -32001: Request timed out                                                                                                                                                                                     │
-                                                                                                                                                                                                                        │
-⚙ patterm_get_process_status [process_id=p_a6726d]                                                                                                                                                                      │
-MCP error -32001: Request timed out                                                                                                                                                                                     │