chore(ai): migrate to dual-agent handoff system

Split the monolithic CLAUDE.md into a durable handoff system:

- .ai/PROJECT_CONTEXT.md  — stable architectural truth (stack, structure,
  SaaS shape, ConnectWise, coding standards, frontend patterns, critical
  lessons). Ported verbatim from the previous CLAUDE.md.
- .ai/CURRENT_TASK.md     — single active task with DoD + out-of-scope.
- .ai/HANDOFF.md          — resume point, kept under ~2K tokens.
- .ai/TODO.md             — backlog, read only when CURRENT_TASK complete.
- .ai/DECISIONS.md        — append-only architectural decision log.
- .ai/SESSION_LOG.md      — append-only chronological history.
- .ai/README.md           — human-facing explanation of the system.

Root agent files share a byte-identical protocol block (verified via diff):

- CLAUDE.md — primary agent, with GitNexus + gstack tooling and the
  Claude Opus 4.7 co-author trailer.
- AGENTS.md — OpenAI Codex resume agent, with grep/rg fallbacks and the
  Codex co-author trailer. Steps in when Claude hits session/weekly
  limits.

Legacy root-level SESSION-HANDOFF.md deleted — superseded by .ai/HANDOFF.md.
It was a self-describing one-off from the Design System v4 migration and
had no external references.

Supersedes previous CLAUDE.md. Old version recoverable via
`git show pre-ai-handoff:CLAUDE.md` (tag points at commit e110fed).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.ai/HANDOFF.md

@@ -0,0 +1,24 @@
+<!-- Keep under ~2K tokens. Old handoffs live in SESSION_LOG.md. Do not let this file accumulate history. -->
+
+# HANDOFF.md
+
+**Last updated:** 2026-04-24 (America/New_York) — initial setup
+
+**Active task:** See [CURRENT_TASK.md](CURRENT_TASK.md). (No real task yet — handoff system just initialized.)
+
+**Branch:** `feat/flowpilot-migration`
+
+**Where I left off:**
+- File: n/a
+- Next intended action: first agent to pick up real work should replace `CURRENT_TASK.md` example and start fresh here.
+
+**Uncommitted state:**
+- `backend/pytest.ini`, `backend/requirements-dev.txt`, `backend/tests/conftest.py`, `backend/tests/test_rls_isolation.py` modified (pre-existing, unrelated to handoff-system migration).
+- Safe to WIP-commit? Ask Michael — these are mid-flight test isolation fixes from `dab740d`.
+
+**Immediate next steps:**
+1. When picking up real work, replace this file's body with the actual resume point.
+2. Keep this file under ~2K tokens. If it grows, move older context into `SESSION_LOG.md`.
+
+**Open questions / blockers:**
+- None.