Use env-paths config directory for credentials and session storage
Migrate vit credential/session persistence out of process.cwd() and into env-paths config storage.\n\n- add env-paths dependency and new src/lib/paths.js helper\n- read/write .env via configPath() in src/lib/env.js\n- read/write bsky_session.json via configPath() in src/lib/oauth.js\n- ensure config dir exists before session writes in set/del\n- switch pds-record to createSessionStore().get(did) instead of direct file reads\n- update oauth success output to print actual saved .env path\n- update stale --did option text to reference saved credentials