feat(tui): add popups for prompt autocomplete triggers

[DarkPhilosophy]

What

We missed the ^, @, and # triggers, and possibly one more, so these autocomplete commands need a separate PR.

This PR is now a self-contained, single-commit change based directly on the latest upstream/main. It does not contain the fork branch history.

It adds the passive cursor-adjacent popup renderer and opt-in settings for:

• slash-command and command-argument suggestions;
• @ file and directory mentions;
• # prompt actions and GitHub references;
• : emoji suggestions.

The missing fourth trigger was :. The ^ model picker remains in the dedicated inline model picker PR (#11958), which has its own model-search UI and behavior.

Why

Autocomplete lists normally allocate layout rows below the editor. Opening or filtering them moves the editor and chat, and can disturb native terminal scrolling. The passive popup renderer paints suggestions over the existing frame without allocating transcript rows or taking focus.

The non-command popup gate is restricted to explicit @, #, and : prefixes. Bare or absolute path completion remains inline. Command arguments remain controlled independently by display.commandSuggestionsPopup.

Testing

• bun test packages/coding-agent/test/command-suggestions-popup.test.ts - 12 pass, 0 fail.
• bun run --cwd packages/tui check:types - passed.
• bun run --cwd packages/coding-agent check:types - passed.
• bun run check:tools - passed.
• Integration coverage uses the real autocomplete provider for @ file mentions, # prompt actions, and : emoji. It verifies that the bordered popup stays above the editor, below-editor content remains below it, and native history does not move.

---

• bun check passes
• Tested locally
• CHANGELOG updated with PR link and contributor attribution

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