fix: scope doctor config checks to the selected rig

[Bella-Giraffety]

Summary

• gt doctor --rig <rig> was reading and fixing types.custom and status.custom against the town beads database instead of the selected rig.
• Resolve the target beads dir from the selected rig and pin BEADS_DIR / BEADS_DOLT_SERVER_DATABASE so bd config runs against the intended database.
• Preserve existing rig-local custom types while adding the required Gas Town ones, instead of overwriting the entire types.custom value.
• Add focused regression tests for rig-scoped reads/writes and preservation of existing custom types.

Validation

• GOTOOLCHAIN=auto go test ./internal/doctor/...

Notes

• This PR is intentionally limited to the rig-scoped doctor config checks.
• It does not broaden doctor scoping in other checks.