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-session-start.md

@@ -0,0 +1,37 @@
+---
+description: Start a work session — read memory files, check project status, report what needs attention.
+---
+
+# Session Start
+
+Read project state and report a summary before beginning work.
+
+## Steps
+
+1. **Read recent activity**
+   - Read `docs/memory/session-log.md` — summarise the last 2-3 sessions
+   - Note any incomplete work or open questions
+
+2. **Check component status**
+   - Read `docs/memory/component-registry.md`
+   - Count components by status: done, in-progress, planned, needs-revision
+   - Flag any components stuck in "in-progress" or "needs-revision"
+
+3. **Check token status**
+   - Read `docs/memory/token-registry.md`
+   - Note any gaps or recently added tokens
+
+4. **Check pending reviews**
+   - Read `docs/reference/retroactive-review-plan.md`
+   - List any tiers or components that haven't been reviewed yet
+
+5. **Check recent decisions**
+   - Read `docs/memory/decisions-log.md`
+   - Note the 3 most recent decisions and their status
+
+6. **Report**
+   Present a brief summary:
+   - Recent activity (1-2 sentences)
+   - Component stats (done/in-progress/planned)
+   - Pending review items
+   - Suggested next actions