Add getRig() helper in rig_helpers.go that encapsulates the common
boilerplate for finding town root, loading rigs config, and retrieving
a rig. This reduces duplication across 5 get*Manager functions:
- getPolecatManager
- getSessionManager
- getCrewManager
- getRefineryManager
- getWitnessManager
Closes gt-7sqi.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>