AGENTS.md: documentation-drift sweep as explicit pre-commit step

Companion to the same addition in the cloud-init and ansible repos.
Caught real drift in those repos in a recent session only because
the user explicitly asked. Codify the sweep with concrete, repo-
specific drift hotspots rather than a vague 'watch for drift' rule
that gets ignored.

Each AGENTS.md addition lists the doc files most likely to fall
behind code changes here, plus a quick-triage one-liner using
'git diff --name-only HEAD | xargs grep -l ...' so the rule is
actionable not aspirational.

AGENTS.md

@@ -35,3 +35,16 @@ Container image that adds pi coding-agent on top of the opencode-devbox base ima
 - Do NOT call `mempalace-toolkit/install.sh` in the Dockerfile — the base entrypoint handles it
 - `NPM_CONFIG_PREFIX=/usr` must be set per-RUN for any build-time `npm install -g` to keep baked binaries off the volume-shadowed path
 - The smoke test threshold is 2200 MB — update if the image legitimately grows past it
+
+## Documentation drift sweep
+
+Before committing any non-trivial change, check that prose still matches code. Drift hotspots in this repo:
+
+- `README.md` — quick-start examples, env-var table, base-image reference (must match `FROM` in `Dockerfile`).
+- `AGENTS.md` (this file) — `Key facts` block (pi binary path, `NPM_CONFIG_PREFIX`, base-image tag), smoke-test threshold number.
+- `CHANGELOG.md` — promote `Unreleased` only on tag, but record post-release fixes in a fresh `Unreleased` block.
+- `DOCKER_HUB.md` — hand-maintained slim Hub description; sync anything user-facing that changes (env vars, run command, base image).
+- `.env.example` — hand-updated, must match Dockerfile/entrypoint env vars.
+- `Dockerfile` `PI_VERSION` ARG default — if you intend to pin (rather than `latest`), bump it on release.
+
+Quick triage: `git diff --name-only HEAD | xargs -I{} grep -l 'thing-you-changed' README.md AGENTS.md DOCKER_HUB.md CHANGELOG.md .env.example`.