fix(terminal): agent pane auto-launch lost under fish + Starship (STA-3417) (#12840)

* fix(terminal): extend the shell-ready startup barrier to fish (STA-3417)

Fish never emitted the OSC 777 shell-ready marker, so agent launch
commands were written into the PTY while fish/Starship were still
initializing: the daemon path wrote them synchronously at session
create and the local path blind-wrote ~30ms after the first output
byte. The command was echoed by the kernel but never executed.

- shell-templates: shared fish --init-command that emits the marker
  once on the first fish_prompt event (the earliest point fish's own
  reader owns the PTY, mirroring zsh's zle-line-init marker)
- daemon shell-ready: fish joins the startup barrier so the launch
  command queues until the marker (timeout fallback unchanged)
- local-pty-shell-ready: fish launch config gains the marker wrapper
- codex-startup-delivery/tui-agent-startup: omp/pi/opencode plans now
  request shell-ready delivery (codex parity) so the SSH renderer path
  also waits for the prompt; plain payload-free codex stays on the
  markerless fast path

* fix(terminal): answer DA1 past the shell-ready barrier

The barrier queues all inbound input until the ready marker, including the
renderer's DA1 reply. A shell that withholds its first prompt until DA1 is
answered — fish waits 10s — therefore never emits the marker that would
release the reply it is waiting for. Measured: 10.37s to launch an agent,
versus 0.35s once the reply lands.

Answer DA1 from the daemon while the barrier holds, writing straight to the
subprocess so the reply bypasses the queue, and consume the query so the
renderer's xterm cannot also reply. Released on ready, timeout, or dispose,
handing DA1 back to the renderer for steady state.

Consolidates the identical DA1 handler the ConPTY override already used.

* fix(terminal): prevent duplicate startup DA1 replies

.github/workflows/pr.yml

@@ -183,8 +183,11 @@ jobs:
         with:
           persist-credentials: false
 
-      - name: Install zsh
-        run: sudo apt-get update && sudo apt-get install -y zsh
+      # Why fish: shell-ready.test.ts gates its live fish test on the binary being
+      # present, so without this the fish barrier is only covered by config-shape
+      # assertions and never actually exercised.
+      - name: Install zsh and fish
+        run: sudo apt-get update && sudo apt-get install -y zsh fish
 
       - uses: ./.github/actions/install-node-dependencies
         with: