feat(deepagents): add delete protocol support (#673)

## Summary

Adds protocol-level delete support for deepagents backends.

## Changes

### deepagents

- Adds `DeleteResult` to the backend protocol exports.
- Adds optional `delete(filePath)` support to v1 and v2 backend
protocols.
- Preserves optional delete implementations when adapting backends to
the v2 protocol.
- Implements `StateBackend.delete()` for zero-argument StateBackend
instances by sending `{ [filePath]: null }` through `__pregel_send`.
- Adds a changeset for the new backend protocol capability.