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.

CHANGELOG.md

@@ -16,6 +16,8 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   over MCP.
 
 ### Fixed
+- Slow MCP tool calls such as `wait_for_pattern` no longer block later
+  tool calls on the same MCP connection.
 - Closing an agent now escalates from SIGTERM to SIGKILL when needed,
   so agents that ignore SIGTERM disappear from the running tab bar
   after one Close action while keeping their exited pane readable.