ui: revert mascot from pixel-art crab to simple 3-line bot

[emal-avala]

Summary

Replaces the welcome-banner mascot (pixel-art crab + shimmer animation) with the simple 3-line robot head that existed before commit aad7893.

Why

• Theme compliance — the crab used hardcoded #A422E1 purple and ignored the user's theme. The bot now uses t.accent, so it matches whatever theme is configured.
• Startup latency — the crab's shimmer played 3 frames at 120ms each, adding 360ms of blocking delay before the prompt appeared. The bot is instant.
• Minimal aesthetic — the rest of the REPL (prompt ❯, dividers, status line) is one-line and subtle. The 14-row crab was visually inconsistent.
• Line count — removes ~245 lines of pixel-art and color plumbing.

The mascot

▐▛██▜▌   Agent Code v…
▝▜██▛▘   model · session …
  ▘▘     /path/to/cwd

Three lines of Unicode half-blocks, rendered with t.accent bold on the top row and t.accent regular on the lower rows.

Cleanup

Also deleted the following dead functions from tui.rs (the crab was the only consumer):

• `render_crab_banner`
• `render_crab_shimmer`
• `render_half_block`
• `rgb_fg`, `rgb_bg`

Test plan

• cargo check / cargo clippy (will be verified by CI)
• Visual check: run the REPL and confirm the bot appears in the active theme's accent color
• Verify no theme (light/dark/custom) causes the bot to render in low-contrast colors

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.