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.

adapters/claude/commands/handoff.md

@@ -0,0 +1,17 @@
+---
+description: End-of-session handoff — verify work is saved, update AGENTS.md durable knowledge and Current state, propose commit and push
+argument-hint: [optional focus notes]
+allowed-tools: Bash(git:*), Read, Edit, Write, Grep, Glob
+---
+
+Run the end-of-session handoff checklist for the repository in the current working
+directory.
+Optional focus notes from me (may be empty): $ARGUMENTS
+
+Your complete handoff guide — the close-out checklist, what belongs in durable knowledge
+versus Current state, and the final report format — is at:
+
+    ~/Projects/standards/guides/handoff.md
+
+Read it in full first, then follow it exactly. If you cannot read that file, stop and
+report precisely that — do not improvise the handoff.