feat(content-node): reprovide only pinned content #65

Summary

Limit what the node announces to what the platform pinned.

Estimated size: S

Proposal

• Set the reprovide strategy to pinned or roots (verify option names for the pinned release).
• Document the trade-off: cached blocks are not announced.

Acceptance criteria

• Only pinned CIDs are announced; documented with the command used to check.

Dependencies

• feat(infra): run Kubo in docker-compose (server profile, pinned version) #59

Part of #43. Ground rules for the content layer: no secrets in the repo, content-layer failures must never block or discard quantum execution, and tests and docs ship with every PR.