feat(ui): start-chat modal and pane close controls, retire the sidebar-swap flow

[yicheng47]

The M3-closing UI task (M4 pull-forward): main's Start Chat modal built natively on the M3.5 runtimes backend, plus main's pane close control — the walking-skeleton sidebar-swap new-chat flow is deleted.

What it does

• start_chat.rs: GPUI modal opened by ⌘T / New tab / empty-pane New chat (preselecting the focused chat's runner, like main): runner and runtime modes with persisted preference, runtime dropdown from the discovery-backed selectable catalog with availability states, model + effort fields per runtime, title auto-derive-until-edited, cwd field with a folder picker — via GPUI's cx.prompt_for_paths (review round 1 caught rfd blocking the main thread inside an entity lease; rfd is fully removed), spawn through the override-aware ops, rename-after-spawn.
• modal_text_input.rs: reusable IME-capable text field (pulse text_input pattern + text_util), the seed of the M4.2 widget set.
• Pane close button per main's ChatPaneGroup semantics, including empty panes; layout-tree rebuild ported with tests.
• The sidebar-swap flow (new_chat_target, begin_pane_chat picker branch) is gone.

Also in this PR

• docs: M3 re-scoped to backend-only (clean M3/M4 cut, Jason 2026-08-18); M4 restructured as a staged UI rebuild (M4.1 app shell → M4.6), with hidden-titlebar and app-icon details pinned.

Verification

• make verify exit 0; terminal fixtures 10/10; 36 runner-app tests.
• Codex-peer review clean after one must-fix round (the rfd main-thread block); reviewer independently re-ran clippy/fmt/tests and confirmed the rfd dependency is fully gone.
• Human smoke pass to follow post-merge (reviewer's two dynamic checks: Browse… while a chat streams output; dropdown clipping when the modal is scrolled) — it doubles as M3.5's deferred verification.

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