feat: add Rosé Pine TUI theme

[Ayush7614]

Summary

• Adds the Rosé Pine theme (/theme rose-pine) — warm muted purple with rose & pine accents (partially addresses New TUI themes (Solarized, Tokyo Night, One Dark, Rosé Pine, …) #39).
• Registers palette + description; unit tests mirror the Tokyo Night theme coverage.
• README theme list updated.

Test plan

• bun run typecheck
• bun test
• bun run build
• In TUI: /theme rose-pine and confirm readable contrast

[github-actions]

🤖 KlaatAI Review Bot (powered by Klaatu, advisory only — a maintainer makes the real call)

Issue match
Partially addresses #39. The issue requests multiple themes; this PR specifically implements the "Rosé Pine" portion.

Test coverage
Tests in src/engine/theme.test.ts verify registration, 256-color slot bounds, and hex formats. However, they miss the issue's explicit guideline to "check contrast" for body/dim text and diffs. An automated contrast check or documented manual verification is missing.

Correctness concerns
In src/engine/theme.ts, headingFg is set to the string "white", while other foregrounds use 256-color integers or hex. This risks rendering failures if the TUI expects strict hex/numeric values. Additionally, the issue's acceptance criteria require a PR screenshot, which is absent.

Verdict
Needs a specific fix first: confirm "white" is valid for headingFg (or switch to hex/number), and the human reviewer must verify the missing screenshot and contrast checks.

_{This is an automated review to help triage faster, not a gate. Nothing here blocks merging.}