Skip external CLI history scan for session-backed desktop apps

workspace_multi_pane_resize_reflow benchmark: ~427ms/frame -> 4.08ms
(p95 488ms -> 4.85ms). SingleSessionApp::new() unconditionally walked
~/.codex/sessions + ~/.claude/projects (up to 256 JSONL files, reading
contents) to build the welcome continuation suggestion. Workspace
rendering builds one ephemeral app per visible surface every frame, so
a multi-pane resize triggered 16 full filesystem scans per frame.

The suggestion is only rendered on the fresh welcome screen (no session),
so gate the scan on !has_session. Also flips passes_workspace_resize_budget,
passes_resize_cpu_budget, and passes_120fps_interaction_cpu_budget to true.

Adds regression test session_backed_app_skips_external_cli_scan.