Fail fast on unhealthy recall eval cases

[sheawinkler]

Summary

• validate saved recall eval case sets before retrieval
• return structured agent remediation for fallback, unscoped, root, or multi-file rollup cases
• expose case_set_health on saved case GET/refresh routes

Verification

• GOCACHE=/Volumes/wd_black/codex_go_cache GOMODCACHE=/Volumes/wd_black/codex_gomod_cache go test . -run 'TestMemoryRecallEvaluateSaved(FailsFastForUnhealthyCaseSet|IsGoNative|ScoresGraphContribution)|TestRecallEvalCasesRefreshUsesLiveFileBackedMemory|TestProxyForwardsBatchAndOpsQueuePaths' -count=1
• GOCACHE=/Volumes/wd_black/codex_go_cache GOMODCACHE=/Volumes/wd_black/codex_gomod_cache go test . -count=1