feat: add AGY (Antigravity) harness adapter, interactive prompts, and slash commands

[Kryvonis]

Summary

Adds support for AGY (Google Antigravity CLI / agy / antigravity) to Collie:

1. Interactive Prompt Up-Leveling (web/src/lib/harness/agy/):

   • Lifts ask_question dialogs, numbered options (1. Option, 1) Option, [1] Option, ( ) 1.Option), and permission/confirmation prompts into native tappable buttons.
   • Handles options with or without explicit footer hint lines.
   • Guards against free-text sends while an interactive prompt is open (composerReady).
   • Re-derives signatures through the race guard before sending keystrokes ([digit, "Enter"] or [digit]).

2. Slash Command Palette (web/src/lib/agent-commands.ts):

   • Adds the public AGY command catalog with prefix matching for agy, agy-cli, and antigravity:
     /plan, /grill-me, /learn, /compact, /clear, /model, /status, /subagents, /skills, /rules, /mcp, /hooks, /review, /diff, /doctor, /theme, /help, /exit.

3. Brand Styling & Icon (web/src/components/agent-icon*):

   • Adds the official brand mark and tile background (#1A73E8) for agy and antigravity pane badges and switcher tiles.

4. Registry & Case/Prefix Normalization (web/src/lib/harness/registry.ts):

   • Normalizes agent keys in adapterFor to support casing and common prefix variants (agy, agy-cli, antigravity).

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Suggested CHANGELOG Entry

AGY (Antigravity CLI) gets a harness adapter — up-levels interactive question options (ask_question) and tool permission dialogs into tappable buttons, gates the composer against typing into active menus, adds the AGY slash command palette, and includes brand icons.

Verification

- **Harness Unit Tests:**
  `cd web && bunx vitest run src/lib/harness/agy/` (7 passed)
- **Commands & Icon Tests:**
  `cd web && bunx vitest run src/lib/agent-commands.test.ts src/components/agent-icon.test.tsx` (39 passed)
- **Full Frontend Suite:**
  `cd web && bun run test` (108 test files, 2,161 tests passing)
- **Bridge Tests:**
  `bun test bridge/` (559 tests passing)
- **Typecheck & Production Build:**
  `cd web && bun run typecheck && bun run build` (0 errors, clean build)
- **Live Local Test:**
  Verified prompt button tapping and submission against a live `agy` session on Herdr.