moltenhub-preserve follow-up task logs after local cleanup

[jefking]

Summary

• archive local task log files to .log/followup/... when building failure/no-changes follow-up prompts
• use archived paths in follow-up prompt generation so cleanup of .log/local/... does not break follow-up log review
• add tests that verify archived paths are used and remain readable after source cleanup

Validation

• go test ./cmd/harness
• go test ./internal/harness
• go test ./internal/hub
• go test ./...
• go test ./internal/harness -run TestRunMultiRepoClonesConcurrently -count=1
• go test ./internal/hub -run 'TestDispatchResultPayloadIncludesFailureMessageAndDetails|TestHandleDispatchQueuesFailureFollowUpWithTaskLogPaths' -count=1
• go test ./cmd/harness -run 'TestFollowUpTaskLogPathsArchivesLocalTaskLogs|TestUnexpectedNoChangesFollowUpRunConfigPreservesTaskTargetingAndAddsContext|TestFailureFollowUpRunConfigUsesRequiredPayloadShapeAndLogContext' -count=1