perf(tui): defer agent construction until first prompt

Match classic CLI perceived startup behavior: show the TUI shell and composer
before constructing the full AIAgent. session.create now returns a lightweight
placeholder session with lazy=true and no longer starts _make_agent eagerly.
The first method that needs the agent triggers _start_agent_build() via _sess();
prompt.submit is routed through the RPC worker pool so that the initial wait for
agent construction does not block the stdio dispatcher.

The intro panel renders skeleton rows for tools/skills while the real
session.info payload is absent, then hydrates to the real tools/skills panel once
AIAgent initialization completes. Also skip the startup /voice status probe and
avoid the input.detect_drop RPC for ordinary plain-text prompts to keep early
startup/first-submit paths cheap.

Measurements on macOS Terminal.app:
- Previous full ready p50 after earlier PR commits: ~1537ms
- Lazy skeleton panel p50: ~794ms
- Original baseline full ready p50: ~1843ms

So the visible startup surface is now ~743ms faster than the prior PR state and
~1.05s faster than the original baseline. First prompt still pays the same agent
construction cost if it races the background/skeleton state, matching classic
CLI's deferred behavior.

Tests:
- python -m py_compile tui_gateway/server.py
- cd ui-tui && npm run type-check && npm run build
- scripts/run_tests.sh tests/tui_gateway/test_protocol.py::test_sess_found tests/tools/test_code_execution_modes.py tests/tools/test_code_execution.py
- cd ui-tui && npm test -- --run src/__tests__/useSessionLifecycle.test.ts src/__tests__/useConfigSync.test.ts

ui-tui/src/app/useSubmission.ts

@@ -126,6 +126,13 @@ export function useSubmission(opts: UseSubmissionOptions) {
         return sys('session not ready yet')
       }
 
+      // Plain prompts are the common path and should not pay an extra RPC
+      // before prompt.submit. File-drop detection can still run for inputs
+      // that contain an absolute/tilde path or file:// URI.
+      if (!looksLikeSlashCommand(text) && !/(?:^|\s)(?:file:\/\/|~\/|\/)[^\s]+/.test(text)) {
+        return startSubmit(text, expand(text), showUserMessage)
+      }
+
       gw.request<InputDetectDropResponse>('input.detect_drop', { session_id: sid, text })
         .then(r => {
           if (!r?.matched) {