refactor(fxt): complete TLI→FXT rename + fix token storage panic
- Rename TliConfig→FxtConfig, TliError→FxtError, TliResult→FxtResult
- Migrate token storage path foxhunt-tli→foxhunt-fxt with auto-migration
- Fix FileTokenStorage::Default panic (fallback to /tmp on missing HOME)
- Update all doc comments, login prompt, config.toml.example, env vars
- Update keyring service names foxhunt-tli-access→foxhunt-fxt-access
- Add TOKEN_REFRESH_BUFFER_SECS named constant
- Add clarifying comments for JWT dummy key and IBKR default client ID
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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