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-build-component.md

@@ -0,0 +1,51 @@
+---
+description: Scaffold and build a new component following FA conventions and the full component lifecycle.
+---
+
+# Build Component
+
+Create a new component following the FA design system conventions and the
+component lifecycle defined in `docs/reference/component-lifecycle.md`.
+
+## Arguments
+
+Provide the tier and component name (e.g., `atom Button`, `molecule PriceCard`).
+
+## Pre-flight
+
+1. Read `docs/memory/component-registry.md` — confirm component is planned, not already in-progress
+2. **Dependency check (molecules):** all constituent atoms must be `done`
+3. **Dependency check (organisms):** all constituent molecules and atoms must be `done`
+4. Read `docs/conventions/component-conventions.md` — follow the template
+5. Read `docs/design-system.md` — understand brand context for this component
+6. Read `docs/memory/token-registry.md` — know available tokens
+
+If any dependency is not met, STOP and report what's missing.
+
+## Build
+
+1. Create folder: `src/components/{tier}/{ComponentName}/`
+2. Create `ComponentName.tsx`:
+   - Extend appropriate MUI base component props
+   - All visual values from theme (never hardcode)
+   - `React.forwardRef` for interactive elements
+   - Forward `sx` prop
+   - JSDoc on every prop
+   - `displayName` set
+3. Create `ComponentName.stories.tsx`:
+   - `tags: ['autodocs']` in meta
+   - Stories: Default, AllVariants, AllSizes, Disabled, Loading, Error, LongContent, MinimalContent
+4. Create `index.ts` barrel export
+5. Create component tokens in `tokens/component/{component}.json` if needed
+
+## Verify
+
+- Open story in Storybook at `localhost:6006`
+- Confirm all stories render without errors
+- Check visual appearance matches design system
+
+## After
+
+- Update `docs/memory/component-registry.md` — mark as `review`
+- Update `docs/memory/token-registry.md` if tokens were created
+- Update `docs/memory/decisions-log.md` with any design decisions