Cryptographic package signing for OKH packages

Context

OKH package integrity is currently based on SHA-256 checksums in metadata/file-manifest.json. The federation layer already has cryptographic signing infrastructure (identity.sign_json in src/core/federation/) and signs catalog records per-manifest, but built packages are not signed.

This is listed as a planned future enhancement in docs/packaging/okh-packages.md.

What to change

• Wire the existing NodeIdentity.sign_json from the federation layer to package signing
• When a package is built, generate a metadata/package-signature.json containing:
  • signed_by: the node's DID
  • signature: hex signature over the canonical file-manifest.json content
  • signed_at: ISO timestamp
  • algorithm: "ed25519" (matching existing federation identity)
• Add ohm package verify-signature <org/name> <version> to validate the signature against the known public key
• Include signature in push/pull workflow (pushed packages carry their signature; pulled packages can be verified)

Acceptance criteria

• Built packages include metadata/package-signature.json
• ohm package verify-signature validates the signature and reports pass/fail
• Packages built without an identity (no federation config) skip signing gracefully with a warning
• Signature is preserved through push/pull round-trips
• Unit tests cover signing and verification; tampered-manifest detection

Dependencies

Depends on federation identity infrastructure in src/core/federation/identity.py (already implemented).

Notes

The federation layer's existing signing scheme uses Ed25519 via the node's DID key. Reuse this directly — do not introduce a second signing mechanism.

Start with src/core/federation/identity.py and docs/packaging/okh-packages.md, then trace the package build, push, pull, and command entry points. Reuse the existing Ed25519 signing flow and add tests for signing, verification, tampered manifests, missing identities, and push/pull preservation. Done means signed packages verify through the new command and unsigned builds warn and continue.