fix: propagate double-tracked inspection errors

[johnson-ricon]

Summary

• make notes stage fail when double-tracked path inspection fails
• make notes status propagate the same safety-check failure instead of reporting zero conflicts
• add CLI-level regressions proving stage leaves the index untouched and status exits non-zero

Context

Focused fix-it for #189, based directly on reviewed head 173a2ee7fe17ad090ed9f83a963d19c7c7dea82b.

Validation

• mise run test changes (20 passed)
• mise run test (438 BATS passed, 9 Python passed; 6 existing skips)
• codebase lint "$PWD" (8 rules passed)
• mise run doctor (passed; existing optional local-hook warning only)
• readme build --check
• bash -n .mise/tasks/stage .mise/tasks/status test/changes.bats
• git diff --check

Local git pre-commit and git pre-push commands are unavailable in this checkout, so direct repository checks were used.