Fix retained progress handling for retryable runtime failures

[yvette-carlisle]

Summary

• stop generic retryable runtime/app-server failures with tracked worktree changes as retained partial progress
• preserve repo-gate continued-repair failures as bounded automatic repair
• document the runtime contract and add a regression test

Root Cause

XY-941 attempt 1 reached validation-ready work with dirty tracked changes, but the retryable failure branch wrote generic retry/idle handling before retained-progress terminal handling could own the lane. That made the lane look abnormally idle instead of a clear manual takeover point.

Validation

• cargo test -p decodex retryable_runtime_failure_with_retained_tracked_changes_records_retained_partial_progress --all-features
• cargo test -p decodex loop_guardrail_does_not_classify_dirty_retained_diff_as_no_effective_diff --all-features
• cargo test -p decodex retryable_failures_ignore_prior_continuation_attempts_in_writeback --all-features
• cargo make fmt
• cargo make lint-fix
• cargo make test
• cargo make check
• git diff --check