fix: wake cmux surfaces in hidden workspaces before sending launch commands

[elucid]

Problem

When a subagent is spawned while the user is viewing a different cmux workspace the launch command isn't evaluated.

Example, if you prompt the main agent, Cmd+2 to another workspace before the subagent pane is created, the new split pane opens, the shell-ready delay passes, and the bash <script> command is sent but not evaluated. You'll see it there when you switch back to the initial workspace.

Root Cause

Creating a fresh cmux surface in a hidden workspace creates the surface but defers terminal/PTY initialization until the surface receives activity or the workspace becomes visible. The first cmux send to the surface triggers initialization, but its payload is consumed as display text rather than shell input — the shell hasn't started yet when the text arrives.

Confirmed via a red/green cmux integration test that uses the same low-level launch shape as subagents (sendLongCommand() → bash <script>), without any LLM call:

• Pre-fix: the hidden split renders bash <script> before the login banner, but the script never executes (TEXT_ONLY)
• Post-fix: the marker emitted by the script appears on screen, proving the command executed

Fix

Layered the wake behavior into the cmux surface creation paths introduced by #36.

After creating and renaming a fresh cmux surface, cmux.ts checks the pre-create cmux identify --json snapshot. If the caller workspace differs from the focused workspace, the caller workspace is hidden, so Pi sends a harmless \n to the newly-created target surface. This forces cmux to initialize the hidden terminal before the existing shell-ready delay sends the real launch command.

The wake is applied in both fresh cmux creation paths:

• createCmuxSplitSurface()
• createSurfaceInPane()

This preserves #36’s focus-neutral behaviour: no focus-pane, no select-workspace, and no replacement of the CMUX_SURFACE_ID anchored creation flow.

[elucid]

Ugh, this is actually no longer working after my rebase onto the changes #36 introduced. I need to go back to the drawing board...