Extract handoff substance into vendor-neutral guide

Move the handoff checklist body into guides/handoff.md as plain prose,
and replace the Claude command with a thin wrapper that reads the guide,
matching the full-eval pattern.

guides/handoff.md

@@ -0,0 +1,46 @@
+# Session handoff
+
+The session is about to end. Run the close-out checklist below. Use judgment — skip any
+step that doesn't apply to what was done this session, and tell the user what you skipped
+and why. The user may provide optional focus notes; weave them in where relevant.
+
+## 1. Verify the work is on disk
+
+- If this is a git repo, run `git status` and report any uncommitted or untracked changes
+  related to this session's work.
+- If meaningful work is uncommitted, propose a commit (or a few logical commits) with clear
+  messages. Wait for the user's approval before committing — do not commit on your own.
+  After committing, push if a remote is configured.
+- If this is not a git repo, list every file created or modified this session and confirm
+  each one is actually written to disk, not just discussed in conversation.
+
+## 2. Update AGENTS.md — durable knowledge only
+
+- Add or refresh only what every future session will need: architectural decisions made,
+  conventions or patterns established, gotchas discovered, key commands, dependency or
+  structure facts learned the hard way.
+- Keep it lean. AGENTS.md loads at the start of every future session, so no session
+  narrative, no play-by-play. If anything in it is now obsolete or contradicted by today's
+  work, fix or remove it.
+- If a piece of guidance is subsystem-specific rather than whole-repo, put it in
+  docs/guides/<topic>.md with paths: frontmatter, symlink it from the harness's rules
+  directory, and add its one-line index entry in AGENTS.md instead.
+
+## 3. Update the Current state section — session state
+
+- Rewrite the `## Current state` section at the bottom of AGENTS.md. It represents current
+  state, not a running log — overwrite, don't append. Present tense, 15 lines max. Cover:
+  - What's done and working, including anything finished this session
+  - What's in progress and exactly where it stands
+  - Next steps in priority order
+  - Open questions, risks, or anything the user flagged that wasn't resolved
+  - Test/build status if worth knowing
+- Anything longer-term goes in ROADMAP.md, not here. If Current state is accumulating
+  history, prune it — history lives in the git log.
+
+## 4. Final report
+
+Reply with a short summary: what got committed or pushed, what went into durable knowledge
+versus Current state, and anything still unresolved. If everything is clean, say it's safe
+to exit. Then, in case the user decides to keep the session alive instead, give them a
+one-line `/compact Focus on ...` command tailored to what matters most from this session.