Add 7 new checks for rig health when using `gt doctor --rig <name>`:
- rig-is-git-repo: Verify mayor/rig/ is a valid git clone
- git-exclude-configured: Check .git/info/exclude has Gas Town dirs (fixable)
- witness-exists: Verify witness/ structure exists (fixable)
- refinery-exists: Verify refinery/ structure exists (fixable)
- mayor-clone-exists: Verify mayor/rig/ clone exists (fixable)
- polecat-clones-valid: Verify polecat directories are valid clones
- beads-config-valid: Verify beads configuration works (fixable)
Checks are only registered when --rig flag is provided.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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