feat: add Sentry error reporting

Capture command errors and panics with Sentry SDK. DSN injected
at build time via ldflags (dev builds silent, releases report).
Opt-out: UNARR_NO_TELEMETRY=1.

.goreleaser.yml

@@ -17,6 +17,7 @@ builds:
     ldflags:
       - -s -w
       - -X github.com/torrentclaw/torrentclaw-cli/internal/cmd.Version={{.Version}}
+      - -X github.com/torrentclaw/torrentclaw-cli/internal/sentry.dsn={{ .Env.SENTRY_DSN }}
 
 upx:
   - enabled: true