Summary

• make Beads town-root detection fall back to GT_TOWN_ROOT/GT_ROOT when cwd is outside the town
• add regression coverage for accent_unified_au MR routing from an external cwd
• add short-prefix hook routing coverage for au-* formula bonding

Verification

• go test ./internal/beads -run 'TestCreateRoutesExplicitRigUsingEnvTownRootFromExternalCwd|TestResolveHookDirUsesShortPrefixRoute|TestCreateRoutesToResolvedRigBeadsDir|TestGetPrefixForRig'
• go test ./internal/cmd -run 'TestMRBeadCreationUsesRig|TestDoneUsesResolveBeadsDir|TestDoneBeadsInitBothCodePaths|TestSlingFormulaOnBeadSetsAttachedMolecule|TestSlingRejectsCrossRigMismatch|TestResolveMQSubmitCommitSHAUsesSubmittedBranch|TestVerifyMQSubmitPushedBranchRequiresRemoteBranch'
• go test ./internal/beads
• go test ./internal/beads ./internal/cmd ./internal/mq (fails in unrelated existing internal/cmd tests: agents, convoy, install)