docs: document snapshot prune + auto-prune env and pool --metrics-addr (#137)

Per CLAUDE.md (docs reflect completed features): the operability batch added
'snapshot prune --keep/--max-bytes' + A3S_BOX_MAX_SNAPSHOTS/_BYTES auto-prune
(bound snapshot disk) and 'pool start --metrics-addr' (Prometheus /metrics for
the warm-pool daemon). README's snapshots + warm-pool sections now show them.

Co-authored-by: Roy Lin <roylin@a3s.box>

Author: ZhiXiao-Lin

README.md

@@ -210,10 +210,13 @@ a3s-box export app -o rootfs.tar
 a3s-box commit app -t app:snapshot
 a3s-box snapshot create app checkpoint-1
 a3s-box snapshot restore checkpoint-1 --name restored-app
+a3s-box snapshot prune --keep 5          # bound disk: keep the 5 newest
 ```
 
 The `snapshot` command produces configuration/filesystem-oriented Box snapshots, not a live RAM checkpoint. The live RAM Copy-on-Write facility is a separate, lower-level mechanism described in [Warm pool and snapshot-fork](#warm-pool-and-snapshot-fork).
 
+Each snapshot deep-copies the box rootfs, so a scheduled snapshot workflow can fill the disk. `snapshot prune --keep N` / `--max-bytes B` evicts the oldest beyond a cap; set `A3S_BOX_MAX_SNAPSHOTS` / `A3S_BOX_MAX_SNAPSHOT_BYTES` to auto-prune on every `snapshot create` (unset = unbounded).
+
 ## Warm pool and snapshot-fork
 
 A **warm pool** keeps a set of sandboxes pre-booted and serves them over a Unix
@@ -225,11 +228,14 @@ json-file console logs).
 ```bash
 a3s-box pool start --image alpine:latest --size 8     # pre-warm 8 sandboxes
 a3s-box pool start --image alpine:latest --size 8 --snapshot-fork   # CoW fill
+a3s-box pool start --image alpine:latest --metrics-addr 127.0.0.1:9101   # + Prometheus /metrics
 a3s-box pool run alpine:latest -- echo hi             # served from the pool
 a3s-box pool status
 a3s-box pool stop
 ```
 
+`pool start --metrics-addr` serves a Prometheus `/metrics` endpoint with warm-pool hit/miss, VM-boot, and cache metrics for the long-running daemon (alongside `monitor --metrics-addr`'s box-state metrics + `/healthz`).
+
 **Snapshot-fork** (`--snapshot-fork`, Linux `/dev/kvm` only) is native
 Copy-on-Write microVM cloning. The pool cold-boots one template sandbox,
 snapshots its file-backed guest RAM together with KVM vCPU and virtio device