feat: add Tokyo Night theme

[floze-the-genius]

Summary

• add a Tokyo Night palette with blue, green, and navy-aligned terminal colors
• register tokyo-night in /theme and document the expanded theme list
• add focused tests for palette registration and valid 256-color slots

Contributes to #39.

Screenshot

Captured from the real TUI at 120x40 using /theme against an isolated localhost health stub.

Contrast checks

Checked against Tokyo Night background #1a1b26 (code foreground against #262626):

• accent: 6.79:1
• secondary text: 4.89:1
• user green: 9.35:1
• body text: 12.25:1
• muted text: 4.98:1
• tool text: 7.44:1
• code text: 6.92:1

Validation

• bun run typecheck
• bun test (63 passed)
• bun run build
• real TUI /theme picker and persistence via isolated config.json

[github-actions]

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

Issue match
Fully addresses the Tokyo Night requirement of #39. It adds the palette, registers it for the /theme command, updates the README, and includes the required screenshot.

Test coverage
src/engine/theme.test.ts verifies theme registration and validates that numeric color slots are valid 256-color integers. However, it explicitly filters out and ignores the hex string values (e.g., accent: "#7aa2f7"). A test validating the hex strings (or confirming the engine accepts mixed hex/integer types) is missing.

Correctness concerns
In src/engine/theme.ts, TOKYO_NIGHT_PALETTE mixes hex strings (accent, dimText, userColor, border) with 256-color integers and bg: null. While this likely matches existing theme patterns, it poses a risk if the TUI renderer expects a uniform type. Additionally, bg: null relies on the renderer correctly defaulting to the terminal's native background without throwing a null reference error.

Verdict
Ready to merge as-is, but flag the mixed hex/integer color types and bg: null handling to a human reviewer to confirm the rendering engine processes them without runtime errors.

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

[aakashrajput]

@floze-the-genius can you take care of the comment.

[floze-the-genius]

Added a focused palette test covering the mixed representation explicitly: terminal ANSI slots remain integer bytes, CSS-style slots are validated as six-digit hex colors, and bg: null is pinned as intentional so the terminal background shows through. bun test src/engine/theme.test.ts and bun run typecheck both pass.

[aakashrajput]

@floze-the-genius
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