feat(ui): add keyboard prompt navigation

[thatdudealso]

Summary

• Add read-only keyboard navigation through user prompts derived from existing transcript entries.
• Bind Ctrl+P to previous prompt and Ctrl+N to next prompt through the raw input path, avoiding composer history recall.
• Keep the transcript at the selected prompt while streaming and flash the target briefly.
• Suppress streaming auto-follow while prompt review is active, clearing the review lock when the user returns to the bottom or sends a new message.
• Keep Ctrl+P/Ctrl+N in exactly one handler so one keypress advances one prompt.

Testing

• bun run format
• bun run typecheck
• bun run lint (passes with five pre-existing unrelated warnings)
• bun test src/ui/prompt-navigation.test.ts (7 tests)
• Manually verified in the real bun run dev TUI with a local streaming API stub: after sending two prompts, one Ctrl+P while the composer was focused moved to the previous prompt, left composer text unchanged, flashed the target, and later stream chunks did not return the view to the bottom.
• Verified the prompt-navigation binding is installed only through prependInputHandler; the duplicate handleKey path is removed.
• No GIF or upstream merge performed.

Closes #345

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Note

Low Risk
Localized TUI scroll and input behavior with dedicated tests; no auth, API, or data-path changes.

Overview
Adds read-only transcript navigation between user prompts with Ctrl+P (previous) and Ctrl+N (next), documented in the README.

Bindings are handled via prependInputHandler on raw input so they do not fight composer history or normal arrow keys. A small prompt-navigation helper derives user-message indices from chat entries and picks the nearest scroll anchor without wrapping.

While you are reviewing a prompt, streaming auto-follow is paused until you scroll back to the bottom, send a message, clear the session, or similar; the selected user message gets a brief accent flash. Unit tests cover index derivation, nearest-prompt lookup, and key mapping.

^{Reviewed by Cursor Bugbot for commit 3f62622. Configure here.}

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