fix(tui): restore slash suggestions from deferred runtime startup

[kingsword09]

Summary

• Fix missing /mcp, /help, /model, and custom-command suggestions with current upstream runtimes: they return commands from loadStartupOptions() rather than top-level options.
• Paint a cancellable startup screen, call the loader once, and merge startup metadata before constructing the editor and session state; preserve compatibility with legacy top-level options.
• Restore terminal state on startup failure or cancellation, ignore late resolution/rejection, and prevent user input from reaching a session before initialization.
• Add isolated real-PTY regression coverage, a real-runtime /mc completion smoke assertion, and local validation documentation.

Validation

• bun run typecheck — passed
• env -u KITTY_WINDOW_ID TERM=xterm bun run test:unit — 852 passed, 2 skipped
• bun run test:tui — 31 component + 34 PTY tests passed; the final startup tests also rerun separately (5 passed)
• bun run test:runtime — 62 passed
• bun run test:node — 20 passed
• bun run sync:local and bun scripts/check-runtime.ts — passed against installed Desktop 3.15.3 / runtime 0.16.9
• env -u KITTY_WINDOW_ID TERM=xterm-256color bun run check:tui — all smoke checks passed, including real-runtime slash completion and built-TUI focus checks
• git diff --check — passed

Environment notes

• Existing notification/fullscreen smoke assertions are sensitive to inherited Kitty detection. They pass with KITTY_WINDOW_ID removed; unrelated notification code/tests were not changed.
• The package provenance check was attempted but rejects the locally synchronized macOS Desktop source because zcode-runtime.lock.json pins the Linux release artifact. The lock is unchanged; CI uses sync:locked for release validation.

Manual verification

1. Run bun run sync:local on macOS with /Applications/ZCode.app installed (otherwise bun run sync:locked).
2. Restart zcode; type /mc, /he, and /mod without submitting and verify runtime command suggestions appear.
3. Submit /mcp and verify the configured server picker opens (or the empty configuration notice appears).
4. Run bun test test/tui/startup.test.ts and bun scripts/smoke-tui.ts for isolated and real-runtime regression checks. These use temporary homes and do not make model API calls.

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