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-preflight.md

@@ -0,0 +1,52 @@
+---
+description: Pre-commit quality checks — TypeScript, ESLint, Prettier, Storybook build, token sync, hardcoded values. // turbo
+---
+
+# Preflight Checks
+
+Run all quality gates before committing. Report PASS/FAIL/WARN for each.
+
+## Checks
+
+1. **TypeScript compilation**
+   Run `npx tsc --noEmit`
+   FAIL if any type errors. Report file and line for each error.
+
+2. **ESLint**
+   Run `npm run lint`
+   FAIL if any errors (warnings are WARN). Report count and top issues.
+
+3. **Prettier**
+   Run `npm run format:check`
+   FAIL if any files need formatting. List files.
+
+4. **Storybook build**
+   Run `npx storybook build --quiet`
+   FAIL if build errors. Report the failing component.
+
+5. **Token sync**
+   Compare timestamps: are any `tokens/**/*.json` files newer than `src/theme/generated/`?
+   WARN if generated files are stale. Fix: `npm run build:tokens`
+
+6. **Hardcoded values scan**
+   Search `src/components/**/*.tsx` (excluding `*.stories.tsx`) for hex colour patterns (`#[0-9a-fA-F]{3,8}`).
+   WARN for each match not on a line containing `// ok-hardcode`.
+
+7. **Component exports**
+   For each component folder in `src/components/`, verify `index.ts` exists and re-exports the component.
+   FAIL if any component is missing its barrel export.
+
+## Report format
+
+| Check | Status | Details |
+|-------|--------|---------|
+| TypeScript | PASS/FAIL | error count |
+| ESLint | PASS/FAIL/WARN | error/warning count |
+| Prettier | PASS/FAIL | files needing format |
+| Storybook | PASS/FAIL | failing component |
+| Token sync | PASS/WARN | stale files |
+| Hardcoded values | PASS/WARN | matches found |
+| Exports | PASS/FAIL | missing index.ts |
+
+If all PASS: safe to commit.
+If any FAIL: fix before committing.