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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-claude-proxy",
-  "version": "3.15.1",
+  "version": "3.16.0",
   "description": "OCP (Open Claude Proxy) — use your Claude Pro/Max subscription as an OpenAI-compatible API for any IDE. Works with Cline, OpenCode, Aider, Continue.dev, OpenClaw, and more.",
   "type": "module",
   "bin": {