feat(deepagents): implement delete across backends (#680)

## Summary

Implements the optional backend `delete` protocol across the built-in
backend implementations now that the delete protocol surface is
available. This keeps delete behavior backend-local and does not expose
a new filesystem tool to agents.

## Changes

### deepagents

- Implements `delete(filePath)` for filesystem, store, composite,
context hub, and sandbox backends.
- Uses backend-specific safety semantics: filesystem deletion goes
through existing path resolution and only unlinks files, store deletion
treats paths as exact keys, context hub deletion commits null entries,
and sandbox deletion uses shell-quoted `rm -f`.
- Adds tests covering successful deletion, missing paths, directory
rejection where applicable, composite route path restoration, store
wildcard literal behavior, and context hub cache updates/failure
handling.

### @langchain/node-vfs

- Implements `delete(filePath)` for the VFS backend.
- Adds tests for file deletion, missing paths, directory rejection, and
preserving non-target files.