image: add banger image cache prune for OCI cache cleanup

OCI layer blobs accumulate forever — every pull writes layers to
~/.cache/banger/oci/blobs/sha256/<hex> via go-containerregistry's
filesystem cache, and nothing ever evicts them. The cache is purely
a re-pull-avoidance (every flattened image is independent of the
blobs that sourced it), so it's a perfect candidate for an opt-in
operator-driven prune.

New surface:
  * api: ImageCachePruneParams{DryRun}, ImageCachePruneResult
    {BytesFreed, BlobsFreed, DryRun, CacheDir}.
  * daemon: ImageService.PruneOCICache walks layout.OCICacheDir for
    a (bytes, blobs) tally, then — outside dry-run — atomically
    renames the cache aside, recreates it empty, and rm -rf's the
    aside dir. The rename-then-rm avoids leaving the cache in a
    half-removed state if a pull starts mid-prune (the in-flight
    pull's open files survive the rename via standard Linux
    semantics; it just sees a fresh empty cache afterwards). Missing
    cache dir is treated as zero — fresh installs that have never
    pulled an OCI image don't error.
  * dispatch: image.cache.prune RPC (paramHandler-wrapped, mirroring
    every other image RPC). Documented-methods test list updated.
  * cli: `banger image cache` group with a `prune` subcommand
    (--dry-run flag). Output is a single line: "freed 1.2 GiB
    across 47 blob(s) in /var/cache/banger/oci" or "would free …".
    formatBytes helper for the size pretty-print.

docs/oci-import.md: replaced the "Tech debt: cache eviction" bullet
with a "Cache lifecycle" section describing the new command and
the in-flight-pull caveat.

Tests: PruneOCICache covers the happy path (real prune empties the
cache, recreates an empty dir, doesn't leak the .pruning- aside),
the dry-run path (returns size, leaves blobs intact), and the
fresh-install path (cache dir absent → zero result, no error).
Smoke at JOBS=4 still green; live exercise against an empty cache
on a system install prints the expected zero summary.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/types.go

@@ -174,6 +174,17 @@ type ImageRefParams struct {
 	IDOrName string `json:"id_or_name"`
 }
 
+type ImageCachePruneParams struct {
+	DryRun bool `json:"dry_run,omitempty"`
+}
+
+type ImageCachePruneResult struct {
+	BytesFreed int64  `json:"bytes_freed"`
+	BlobsFreed int    `json:"blobs_freed"`
+	DryRun     bool   `json:"dry_run"`
+	CacheDir   string `json:"cache_dir"`
+}
+
 type ImageListResult struct {
 	Images []model.Image `json:"images"`
 }