XY-856: Add loop guardrail policy and failure-attribution stops

[yvette-carlisle]

Summary

• add persisted loop guardrail checkpoints with normalized failure-attribution stops for repeated validation failures, unchanged deltas, no effective diff, dependency-program stale blockers, review churn aliases, uncovered direction, and ambiguous retained progress aliases
• route guardrail threshold hits through the existing human-required failure/manual-attention path while preserving retained worktrees and private evidence
• surface guardrail reason codes in operator status and update specs/runbooks with recovery guidance

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test (1008 tests run: 1008 passed, 1 skipped)
• git diff --check