Add a standalone recovery tool at `examples/delta-channel-dump/` for
operators rolling back from langgraph >= 1.2 / deepagents 0.6.x to an
older runtime that does not understand `EXT_DELTA_SNAPSHOT` msgpack
blobs. Without this, channels like `messages` can appear empty after
rollback because the old reducer does not decode delta snapshots.
`dump.py` connects directly to Postgres, walks the checkpoint parent
chain (mirroring `aget_delta_channel_history`), decodes msgpack blobs
via `ormsgpack` with a hand-rolled EXT 0–7 hook (no langgraph imports),
and emits JSON with per-channel `seed` + oldest-to-newest `writes`. The
operator reduces and re-applies via `update_state` manually.
## Scope
- Postgres checkpointer only
- Deps: `psycopg[binary]`, `ormsgpack` (not added to repo
`pyproject.toml` — operator installs at runtime)
- No reducer application, no DB writes, no AES/custom encryption support
(fails loudly on encrypted blob types)
## How verified
- E2E against local Postgres: 15 runs of `delta_channel_messages_freq`
(`snapshot_frequency=10`)
- Asserted `delta_kind == "snapshot"`, `len(seed) == 10`, `len(writes)
== 5`, message ids `ai-0`..`ai-14`
- Confirmed portability: fresh venv with only `pip install
"psycopg[binary]" ormsgpack` produced identical output
