feat(volume): disky can't report its own free space

[biliboss]

Problem

An agent guarding "keep at least 30 GB free" cannot get that number out of disky.

• disky stats reports what a past scan measured. Different question, and it needs a snapshot (~3 min traversal) to answer at all.
• disky predict returns fill_at: null with reason provide --free-bytes unless the caller shells out to df -k / | awk '{print $4*1024}' and does the unit math by hand. The default-path output is inert.
• disky growth --over-n's --fill-target help already claims "Default: free bytes on the volume that holds $HOME". The code never implemented it — the flag is a plain Option<u64> passed straight through as None.

So the tool that exists to answer "am I running out of disk" is the one tool in the loop that has to be told how much disk is left.

Proposal

1. volume module — statvfs(2) probe: probe(path) -> VolumeRecord, free_bytes(path) -> Option<u64>.
2. disky free [PATH] — live total / free / used / used_pct for the volume holding PATH (default $HOME), kind="volume", no snapshot required.
3. Default predict --free-bytes and growth --fill-target to the probe, making the existing --help promise true.

Details that matter

• free_bytes from f_bavail (unprivileged-writable), not the root-only figure — the honest answer to "can I still write files".
• f_frsize is the multiplier, not f_bsize (preferred I/O size is the wrong unit for these block counts).
• free_bytes() returns None on probe failure, never 0 — a failed statvfs reporting 0 would read as a full disk and could trigger a cleanup.

Verification

Positive control against df -k at the same instant, plus a negative control (bad path must error, not report 0 free).