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Legacy root-level SESSION-HANDOFF.md deleted — superseded by .ai/HANDOFF.md.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

@@ -0,0 +1,31 @@
+# DECISIONS.md
+
+> Append-only architectural decision log. Newest entries at the top.
+> Entry format:
+>
+> ```
+> ## YYYY-MM-DD — <short title>
+> **Context:** why this came up
+> **Decision:** what we chose
+> **Rejected:** what we didn't choose and why
+> **Consequences:** what this means going forward
+> ```
+
+---
+
+## 2026-04-24 — Adopt dual-agent handoff system (`.ai/` + `CLAUDE.md` + `AGENTS.md`)
+
+**Context:** Claude Code hits session and weekly usage limits. Work stalls when the primary agent is locked out. Needed a structured way for OpenAI Codex to resume where Claude left off without losing architectural truth or drifting across sessions.
+
+**Decision:** Split the old CLAUDE.md into `.ai/PROJECT_CONTEXT.md` (stable repo truth), agent-specific root files (`CLAUDE.md`, `AGENTS.md`) with a shared protocol block, and a small handoff toolkit (`CURRENT_TASK.md`, `HANDOFF.md`, `TODO.md`, `DECISIONS.md`, `SESSION_LOG.md`, `README.md`). Previous CLAUDE.md snapshotted in commit `e110fed` before the migration.
+
+**Rejected:**
+- Single symlinked CLAUDE.md/AGENTS.md — diverges silently, hides agent-specific tooling differences.
+- Putting GitNexus/gstack content in AGENTS.md — Codex doesn't have those tools; would mislead the resume agent.
+- Keeping the old CLAUDE.md as-is and adding AGENTS.md alongside it — duplicated truth, drift guaranteed.
+
+**Consequences:**
+- First read for either agent: `.ai/PROJECT_CONTEXT.md` + `.ai/CURRENT_TASK.md` + `.ai/HANDOFF.md`.
+- Architectural changes in the repo require updating PROJECT_CONTEXT.md, not the root agent files.
+- Git trailers differ per agent (`Claude Opus 4.7` vs `Codex`) — preserved in each root file.
+- Legacy `SESSION-HANDOFF.md` deleted in the same commit; superseded by `.ai/HANDOFF.md`.