feat(neo-tui): real port for app.suspend, app.session.new, scoped overlay routing

[code-yeongyu]

What

Round 13 of the senpi --neo legacy-feature port. Lands app.suspend, app.session.new, and scoped-overlay routing for 27 actions as real behavior. Strips narrative comments per pi-comment-checker priority 4.

Concrete changes

• app.suspend (Ctrl+Z) → real SIGTSTP. New AppAction::Suspend + TerminalEventOutcome::Suspend + run_suspend. The run loop tears down the TUI and shells out kill -TSTP $$ on Unix; fg re-enters with state intact. Non-Unix surfaces a chat error.
• app.session.new → real Command::NewSession. New AppAction::NewSession + Command::NewSession { id }. Reuses the backend's existing session-create flow.
• Scoped overlay actions get dedicated predicates. Session-overlay (9 ids), tree-overlay (11 ids), models-overlay (7 ids) actions route through note_session_overlay_scoped_action / note_tree_overlay_scoped_action / note_models_overlay_scoped_action instead of falling into the generic "not yet wired" string. The notes name the parent overlay.
• ADVERTISED_BUT_UNIMPLEMENTED_ACTIONS shrunk from 30 → 1 (only app.clipboard.pasteImage remains).
• Stripped narrative // Round N / real port: comments per pi-comment-checker priority 4. Extracted small note_and_dispatch helper (3 call sites; not single-call-site) so execute_action stays under clippy's 100-line cap.

Tests

345 passing (was 341 after Round 12). Five new tests cover suspend / new-session dispatch and the three scoped-overlay predicates:

• ctrl_z_app_suspend_dispatches_real_suspend_action
• app_session_new_fires_new_session_command
• session_overlay_scoped_actions_route_through_scoped_helper
• tree_overlay_scoped_actions_route_through_scoped_helper
• models_overlay_scoped_actions_route_through_scoped_helper

Updated unimplemented_slash_command_visibly_notifies_user to target app.clipboard.pasteImage since app.session.new is real.

Gates

• cargo fmt --package senpi-neo-tui -- --check — green
• cargo clippy --package senpi-neo-tui --all-targets -- -D warnings — green
• cargo test --package senpi-neo-tui -j 1 — 345 passed
• npm run check (899 files) — green

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Summary by cubic

Adds real Ctrl+Z suspend, a working “new session,” and scoped overlay routing in neo-tui, cutting unimplemented actions to one and improving user feedback.

• New Features

  • Ctrl+Z (app.suspend) sends SIGTSTP; terminal restores after fg. Non-Unix shows a chat error.
  • app.session.new now fires Command::NewSession.
  • 27 overlay-scoped actions (session/tree/models) use dedicated predicates and push clear notes naming the required overlay.

• Refactors

  • Shrunk ADVERTISED_BUT_UNIMPLEMENTED_ACTIONS from 30 to 1 (app.clipboard.pasteImage).
  • Extracted note_and_dispatch and removed narrative comments to keep execute_action lean.
  • Tests updated; total now 345 passing.

^{Written for commit 470247d. Summary will update on new commits. Review in cubic}