Get Frame At

Resolve a LOGICAL frame ref ({captured_at_ms}) to the stored frame — the FrameSink resolver seam (docs/intent-streaming.md). Returns a 302 to a freshly-presigned S3 URL, so references embedded in copied markdown stay valid: the presign happens per-fetch, never baked into the artifact. Capability-URL semantics (same as the join link). Nearest-match within ±2s tolerates the agent's capture-vs-persist timing skew.