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Legacy root-level SESSION-HANDOFF.md deleted — superseded by .ai/HANDOFF.md.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/TODO.md

@@ -0,0 +1,12 @@
+# TODO.md
+
+> Backlog of work NOT currently active. Read only when `CURRENT_TASK.md` status is `complete`.
+> Format: `- [ ] short description — optional link to issue/PR`
+
+## Up next
+
+- [ ] (seed entry — replace with real next-up items)
+
+## Backlog
+
+- [ ] (seed entry — replace with real backlog items)