feat(snapshot): gcSnapshots + ocp update --rollback --gc + auto-GC; v3.16.0 (#94)

Adds snapshot garbage collection with retention policy: keep last 5,
keep snapshots within 30 days, always keep the most recent. Configurable
via keepCount / keepDays opts.

Wire-up:
- ocp update --rollback --gc (manual trigger; --dry-run supported)
- runFullUpgrade auto-GC after successful upgrade (best-effort,
  swallows errors)

4 unit tests: keepCount enforcement, keepDays override, never-delete-
most-recent safety, dry-run mode.

Bumps to v3.16.0 (bundles PR #93 --check oauth + this GC feature).

No cli.js citation needed: this is OCP-internal snapshot lifecycle with
no corresponding cli.js operation.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

ocp

@@ -709,6 +709,8 @@ Usage:
   ocp update --rollback --list        List available snapshots
   ocp update --rollback <path>        Restore a specific snapshot
   ocp update --rollback --dry-run     Preview rollback plan
+  ocp update --rollback --gc          Delete old snapshots (keep last 5, or <30 days)
+  ocp update --rollback --gc --dry-run    Preview what would be deleted
 EOF
 }