Remove #[cfg(test)] gate from session_control module — SessionStore
is now available at runtime, not just in tests. Export SessionStore and
add workspace_sessions_dir() helper that creates fingerprinted session
directories per workspace root.
This is the #41 kill shot: parallel opencode serve instances will use
separate session namespaces based on workspace fingerprint instead of
sharing a global ~/.local/share/opencode/ store.
The CLI already uses cwd/.claw/sessions/ (sessions_dir()), and now
SessionStore::from_cwd() adds workspace hash isolation on top.