feat: Add malysis cache implementation with proxy flow integration (#346)

* feat: Add malysis cache implementation with proxy flow integration

* fix: Code review fixes

docs/caching.md

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+# Caching
+
+PMG can cache package analysis results across runs to reduce repeat-install
+latency. Caching is opt-in and disabled by default.
+
+## Storage
+
+PMG's on-disk caches share a single SQLite database at
+`<cache-dir>/localdb/pmg.db`, where `<cache-dir>` follows the platform
+convention:
+
+| Platform | Default cache dir |
+|----------|-------------------|
+| macOS    | `~/Library/Caches/safedep/pmg` |
+| Linux    | `$XDG_CACHE_HOME/safedep/pmg` (or `~/.cache/safedep/pmg`) |
+| Windows  | `%LOCALAPPDATA%\safedep\pmg` |
+
+Override the location with the `PMG_CACHE_DIR` environment variable. The cache
+is local to the machine and safe to delete at any time.
+
+## Malysis analysis cache
+
+Persists **benign** Malysis (malware analysis) verdicts on disk, so a package
+version that was already screened is not re-analyzed on every install.
+
+Only benign verdicts are cached, and each entry expires after a TTL. Malicious,
+suspicious, and tenant-excluded verdicts are never cached and are always
+re-evaluated. Keep the TTL below your `dependency_cooldown` window so newly
+published packages remain covered by cooldown if a cached verdict goes stale.
+
+### Enable
+
+In `config.yml`:
+
+```yaml
+analysis_cache:
+  malysis:
+    enabled: true
+    ttl: 24h
+```
+
+Or from the CLI:
+
+```bash
+pmg config set analysis_cache.malysis.enabled true
+pmg config set analysis_cache.malysis.ttl 24h
+pmg config edit   # open the config file directly
+```
+
+- `enabled` (default `false`) — turn the persistent cache on.
+- `ttl` (default `24h`) — how long a cached benign verdict is reused, measured
+  from when it was fetched. A non-positive value disables persistence.
+
+### Manage
+
+```bash
+pmg setup cache status   # show path, enabled state, TTL, and entry count
+pmg setup cache clear    # delete all cached verdicts
+```