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-visual-qa.md

@@ -0,0 +1,40 @@
+---
+description: Browser-based visual QA of a component or page in Storybook. Checks spacing, alignment, states, responsive behaviour.
+---
+
+# Visual QA
+
+Open a component or page story in Storybook and perform a visual quality check
+at multiple breakpoints.
+
+## Arguments
+
+Provide the component name or story path (e.g., `ProvidersStep`, `Button`, `Pages/HomePage`).
+
+## Steps
+
+1. **Ensure Storybook is running** at `localhost:6006`
+
+2. **Navigate to the story**
+   Construct URL: `http://localhost:6006/?path=/story/{tier}-{componentname}--default`
+   If the component is a page: `pages-{pagename}--default`
+
+3. **Check at three breakpoints**
+   For each of mobile (375px), tablet (768px), desktop (1280px):
+
+   - **Spacing:** consistent use of the spacing scale, no visual crowding or excessive gaps
+   - **Alignment:** elements aligned to grid, no off-by-one shifts
+   - **Typography:** hierarchy readable, line lengths between 45-75 characters, weights distinct
+   - **Colour:** all values appear to come from the design system (warm golds, neutral greys, copper accents)
+   - **Interactive states** (if applicable): hover, focus, disabled look correct
+   - **Overflow:** no content clipping, horizontal scroll, or text truncation issues
+   - **Touch targets:** buttons and interactive elements appear at least 44px at mobile
+
+4. **Screenshot each breakpoint**
+   Capture and present to user for review.
+
+5. **Report findings**
+   List issues found with severity:
+   - **P0:** Broken layout, missing content, inaccessible controls
+   - **P1:** Spacing inconsistency, contrast failure, overflow at one breakpoint
+   - **P2:** Minor alignment, typography weight, polish items