feat: add `filecoin-pin get` retrieval command

Description

After a successful upload, filecoin-pin gives the user a CID and retrieval URLs but has no way to fetch content itself. Users must switch to Kubo, a browser, or curl to retrieve what they just stored. This breaks the upload-store-retrieve loop. A tool that can pin should also be able to retrieve. SP retrieval URLs work over plain HTTP and don't require IPFS routing, so a get command doesn't depend on IPNI being ready.

Proposed solution

Add filecoin-pin get <cid>/<pieceCid> that fetches content from the SP retrieval URL. Support -O <filename> to write output to a file (mirroring curl -O). Accept both IPFS CID and Piece CID. Default to stdout so it composes with pipes.

Done criteria

• filecoin-pin get <ipfs-cid> streams file content from the SP retrieval URL
• filecoin-pin get <cid> -O <filename> writes output to disk
• Works without IPNI being ready (fetches directly from SP, not via gateway)
• Piece CID accepted as an alternative to IPFS CID
• Error message is actionable when CID is not retrievable

Notes

• Implementation: wrap verified-fetch