fix: use workflow db home fallback in cloud

[khaliqgant]

Summary

• add cloud/relayfile environment detection for workflow DB home fallback
• enable that fallback from both WorkflowBuilder.run() and the workflow CLI
• cover the detector in file-db.test.ts

Why

Cloud and relayfile-mounted workspaces can make .agent-relay/workflow-runs.jsonl read-only. The JSONL DB already has a home fallback; the builder and CLI just were not opting into it, so cloud runs lost resume state with EACCES warnings.

Validation

• npm --prefix packages/sdk run check
• cd packages/sdk && npx vitest run src/__tests__/file-db.test.ts

Sequence

Independent relay-side fix. It complements the cloud filesystem PRs by preserving workflow run state outside the mounted workspace.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Replay primary run DB before enabling home fallback

Turning on homeFallback at construction time causes relayfile/cloud sessions with a readable-but-read-only workspace DB to switch to $HOME immediately, and JsonFileWorkflowDb only replays the resolved path (loadSnapshot(this.filePath)). In that case, existing runs already stored in .agent-relay/workflow-runs.jsonl are no longer visible, so --resume <runId> can fail with "not found" even though the run state is still present in the original file.

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