[SDK-002] Add Ed25519 signing module

[AdeGneus]

Context

The CLI and Skills Hub need one SDK-owned implementation of Ed25519 signature parsing and verification so signing behavior does not drift across repos.

Scope

• Create ori_sdk/signing.py.
• Implement verify_skill_signature, sign_skill, and BUNDLED_SENTINEL = "bundled".
• Use cryptography for Ed25519 operations.
• Add test vectors from ori-specs/signing/v1.md.
• Expose typed SignatureVerificationError.

Non-Goals

• Does not implement the Hub publish endpoint.
• Does not accept bundled for community-distributed skills.
• Does not use PyNaCl.

Technical Specification

verify_skill_signature(tarball_bytes: bytes, signature_str: str, public_key_b64: str) -> bool verifies ed25519:<base64> signatures. sign_skill(tarball_bytes: bytes, private_key_bytes: bytes) -> str returns the same format. Missing cryptography must fail clearly.

Acceptance Criteria

• Uses cryptography, not PyNaCl.
• Invalid prefix/base64/signature raises typed errors.
• Spec test vectors pass.
• pytest, mypy, and ruff pass.

Tests Required

Test	Verifies
test_verify_signature_vector	Spec vector verifies.
test_rejects_bad_prefix	Non-ed25519 signature rejected.
test_sign_skill_round_trip	Generated signature verifies.

Additional Test Coverage / Edge Cases

• Verify known-good Ed25519 test vectors from ori-specs/signing/v1.md.
• Reject malformed signature prefixes, invalid base64, wrong public-key length, and tampered payload bytes.
• Confirm signatures generated by sign_skill verify with verify_skill_signature.
• Confirm signatures from cryptography are interoperable with runtime verification expectations.
• Confirm bundled is treated as a sentinel value only where explicitly allowed, not as a valid community signature.
• Test missing optional dependency/error path if cryptography import is unavailable or mocked unavailable.

Invariants — Do Not Violate

• Security-sensitive Ed25519 work uses cryptography for runtime interoperability.

Dependencies

Blocked by:

• [SDK-001] Add typed SkillPackage models #3 — SDK-001 Add typed SkillPackage models
• ori-specs signing/v1.md

Unblocks:

• [SDK-003] Add Hub API client #5 — SDK-003 Add Hub API client
• [CLI-008] Implement skills install through Hub client ori-cli#10 — CLI-008 Implement skills install through Hub client

Contract References

• ori-specs/signing/v1.md
• Runtime ori/skills/signing.py

Priority

poc-critical

Suggested Labels

blocked, poc-critical, security-sensitive