The headless guide teaches a chained do command the default belt refuses

Start at docs/HEADLESS.md:561 and compare its chained do command with cmd/codeaf/do.go:3430 and runRoadRefusesStore. Run the deterministic unset-belt and node-belt commands in a throwaway repository, then verify the complete documented chain produces persisted state with exit code 0. Done means the guide states which belt each command demonstrates, a docs command check catches the invalid unset-belt combination, and the change entry records the default-belt invalidation.

Found on santos/dev2 at 008363c98 (#1410). It reaches dev when #1410 merges.

What happened

The section for a sequence of tasks that remember one another tells a reader to run codeaf do "$task" --db "$RUN/store/graph.db" --dir "$RUN/repo" …. On the default belt, --db exits 1 with --db names a store only the older engine works in; … Drop --db, or set CODEAF_TASK_BELT=node to run this on the older engine. No change entry's invalidates tells readers that the documented default changed.

Replication

Deterministic (no model). Create a throwaway repository with R=$(mktemp -d) && git -C "$R" init -q; run codeaf do noop --dir "$R" --db "$R/graph.db" --json with CODEAF_TASK_BELT unset. It exits 1 at flag validation. Set CODEAF_TASK_BELT=node and the flag passes validation. Use OPENROUTER_API_KEY=<any non-empty value> only if continuing to a model call.

Where

docs/HEADLESS.md:561 supplies the command; cmd/codeaf/do.go:3430, runRoadRefusesStore, supplies the refusal at 008363c98.

The fix

Make the guide explicitly select the older engine for durable --db chains, or provide an equivalent supported chain for the default belt. State which behavior each command demonstrates.

Acceptance

• e2e: a reader can copy the guide's complete chain into a fresh repository and receive the documented persisted state, with exit code 0.
• Unit: a docs command check catches an unset-belt invocation combining do and --db.
• Update the manual's headless account and add the default-belt change to a change entry's invalidates.