Add download/verify commands for file retrieval and verification

[timfong888]

Problem

After upload, agents need to verify files are stored and retrievable. Upload response includes pieceScannerUrl and retrievalUrl, but there's no CLI command to act on them.

Details

• No foc-cli download <cid> or foc-cli verify <datasetId> command
• Skill doesn't explain how retrieval works or what retrievalUrl returns
• Agents can't programmatically verify that stored data is actually retrievable (a CID should be part of the experience since it enforced the differentiation)
• This is critical for dApps that need to confirm storage before showing success to users -- (meaning: this is like an acceptance test tha something stored works)

Ask

• Add foc-cli download <cid> — retrieve a stored file by CID
• Add foc-cli verify <datasetId> — check storage status and retrievability
• Document the retrieval flow (what URLs return, expected response format)

[timfong888]

i reviewed this as a human