`agentfs snapshot` command

[penberg]

When an agentfs is mounted with FUSE, for example, you might want to take a snapshot of the filesystem while it's being used.

[bitloi]

Hey @penberg, The snapshot functionality has been implemented in #268. This adds:

• snapshot() method to the SDK
• agentfs snapshot <ID_OR_PATH> <DEST_PATH> CLI command

You can snapshot an active session's database by pointing to its delta.db file:

agentfs snapshot ~/.agentfs/run/<session>/delta.db /path/to/snapshot.db

The implementation uses WAL checkpoint + file copy. Note that there's a small race window between checkpoint and copy where concurrent writes from the FUSE process might not be captured. For fully consistent snapshots of an active mount, a future improvement could have the FUSE process handle the snapshot internally.