Add anonymous command-level analytics to browseros-cli using the PostHog
Go SDK. Tracks which commands are executed, their success/failure status,
and duration — no PII or person profiles.
- New analytics package with Init/Track/Close singleton
- Distinct ID resolves from server's browseros_id (server.json), falls
back to CLI-generated UUID (~/.config/browseros-cli/install_id)
- API key injected at build time via ldflags (dev builds = silent no-op)
- Server now writes browseros_id into server.json for cross-surface
identity correlation
