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Legacy root-level SESSION-HANDOFF.md deleted — superseded by .ai/HANDOFF.md.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

AGENTS.md

@@ -0,0 +1,61 @@
+# AGENTS.md — ResolutionFlow
+
+You are OpenAI Codex, the resume agent for ResolutionFlow. Claude Code is the primary coding agent; you step in when Claude hits session or weekly limits.
+
+The first thing to do every session: read [`.ai/PROJECT_CONTEXT.md`](.ai/PROJECT_CONTEXT.md), [`.ai/CURRENT_TASK.md`](.ai/CURRENT_TASK.md), and [`.ai/HANDOFF.md`](.ai/HANDOFF.md). The ritual is spelled out below.
+
+> The protocol section below is byte-identical to the shared block in CLAUDE.md. If you edit one, edit the other.
+
+## Shared protocol
+
+### Startup ritual (every session)
+
+1. Read `.ai/PROJECT_CONTEXT.md` — architectural truth for this repo.
+2. Read `.ai/CURRENT_TASK.md` — what we're actively working on.
+3. Read `.ai/HANDOFF.md` — exact resume point.
+4. Skim `.ai/DECISIONS.md` for recent entries relevant to the current task.
+5. Run `git log --oneline -15` and `git status`.
+6. Before taking action, state back in two sentences: the current goal and your proposed next action.
+
+### Handoff ritual (session end — limit hit, task complete, or user stop)
+
+1. Update `.ai/HANDOFF.md` to reflect new state. Keep it under ~2K tokens.
+2. If `CURRENT_TASK.md` status changed, update it.
+3. If you made an architectural decision, append to `.ai/DECISIONS.md`.
+4. Append a session entry to `.ai/SESSION_LOG.md`.
+5. If working tree is dirty, commit WIP with `wip(handoff): <one-line summary>`. Do not push unless explicitly asked.
+
+### Writing rules for .ai/ files
+
+- Use model-neutral voice in `HANDOFF.md`, `SESSION_LOG.md`, `DECISIONS.md` ("previous session did X", NOT "Claude did X" or "Codex did X"). Exception: `SESSION_LOG.md` entries include an `<agent>` field in the header.
+- Do not duplicate content between files. `CURRENT_TASK.md` holds the goal, `HANDOFF.md` holds the resume point, `TODO.md` holds the backlog. If unsure where something goes, check `.ai/README.md`.
+- Don't invent facts about the repo. If you're uncertain, write `TODO: confirm` and flag it.
+
+### Project principle
+
+Prefer correct architecture over minimal diff. Flag "simpler approach" tradeoffs for review before taking them.
+
+## Codex-specific notes
+
+### Tooling you do NOT have
+
+- **No GitNexus tools.** Use `grep -r`, `rg`, `git grep`, or `find` for code search. For blast-radius reasoning, grep call sites manually and read the files.
+- **No gstack slash commands** (`/review`, `/ship`, `/qa`, `/browse`, `/investigate`, `/design-review`, `/plan-*`). Run the equivalent work directly: `pytest` for tests, `npm run build` for frontend validation, manual PR description for review flow.
+- **No `/codex` second-opinion command.** You are Codex.
+
+### Git trailer
+
+Every commit: `Co-Authored-By: Codex <noreply@openai.com>`
+
+### Model selection
+
+Handled on OpenAI's side. Do not attempt to set Anthropic model aliases for your own runtime. (The repo's application code still uses Anthropic aliases like `claude-sonnet-4-6` via `settings.get_model_for_action()` — that's runtime config for the product, not your agent.)
+
+### Reviewing Claude's work
+
+When you resume from a Claude session, assume some decisions may have been informed by GitNexus queries or gstack commands whose output isn't in the handoff. If a decision looks unverified from the `.ai/` files alone, either:
+
+- re-verify with `grep`/`rg`/file reads, or
+- flag it in `HANDOFF.md` under "Open questions" so Michael or Claude can confirm on the next handoff.
+
+Do not assume tooling output that isn't written down.