Add cross-tool workflow: AGENTS.md, Antigravity workflows, documentation

- Create AGENTS.md (90 lines) as shared foundation for Claude Code + Antigravity
- Slim CLAUDE.md from 113 to 45 lines (Claude-specific only, references AGENTS.md)
- Slim GEMINI.md from 58 to 26 lines (Antigravity-specific only, references AGENTS.md)
- Add 6 Antigravity workflows in .agent/workflows/ (session-start, preflight, token-sync, visual-qa, build-component, page-review)
- Add docs/reference/cross-tool-workflow.md with task routing, quality gates, file ownership, error mitigation
- Zero content overlap between CLAUDE.md and GEMINI.md (was ~80%)
- File ownership boundaries defined for current phase and future backend work

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.agent/workflows/fa-token-sync.md

@@ -0,0 +1,28 @@
+---
+description: Rebuild CSS and JS outputs from token JSON sources. // turbo
+---
+
+# Token Sync
+
+Validate token JSON and regenerate all outputs.
+
+## Steps
+
+1. **Validate token files**
+   Read each file in `tokens/primitives/`, `tokens/semantic/`, `tokens/component/`.
+   Every token object must have `$value`, `$type`, and `$description`.
+   Report any tokens missing required fields.
+
+2. **Build tokens**
+   Run `npm run build:tokens`
+   Report success or failure with error details.
+
+3. **Verify outputs**
+   Confirm these generated files exist and are non-empty:
+   - `src/theme/generated/tokens.css`
+   - `src/theme/generated/tokens.js`
+   Report any missing or empty outputs.
+
+4. **Check theme integration**
+   Read `src/theme/index.ts` — are any newly added tokens missing from the MUI theme mapping?
+   Report tokens that exist in generated output but aren't consumed by the theme.