feat: sign and verify skill images with Sigstore/Cosign

Summary

Implement cryptographic signing and verification of skill images using Sigstore/Cosign, enabling supply chain security for distributed skills.

Motivation

Unsigned skill registries are a supply chain risk. The ClawHavoc incident (Feb 2026) saw 341 malicious skills on ClawHub — signing is essential for trust in distributed skill ecosystems.

Users need to:

• Sign skill images when publishing to registries
• Verify signatures when pulling/installing skills
• Trust that a skill hasn't been tampered with after publication

Current state

The design docs (docs/design/2026-04-15-oci-skill-registry-design.md, lines 757–961) have extensive specifications for RHTAS integration, but nothing is implemented:

Component	Status
pkg/verify/ package	Planned in CLAUDE.md, directory doesn't exist
skillctl sign command	Not implemented
skillctl verify command	Not implemented
sigstore-go dependency	Not in go.mod
Cosign integration	Not started

Proposed CLI

# Sign a skill image (keyless, OIDC-based)
skillctl sign quay.io/skillimage/summarize:v1.2.0

# Sign with a key pair
skillctl sign --key cosign.key quay.io/skillimage/summarize:v1.2.0

# Verify a skill image
skillctl verify quay.io/skillimage/summarize:v1.2.0

# Verify with a specific public key
skillctl verify --key cosign.pub quay.io/skillimage/summarize:v1.2.0

# Pull with mandatory verification
skillctl pull --verify quay.io/skillimage/summarize:v1.2.0

Design (from existing specs)

Signing flow

1. User publishes a skill image with skillctl push
2. User signs with skillctl sign (or signing happens automatically during promote to published state)
3. Signature is stored as a Cosign-compatible OCI artifact alongside the image

Verification flow

1. During skillctl pull or skillctl install, optionally verify signature
2. Check signature against Rekor transparency log (keyless) or provided public key
3. Reject or warn on unsigned/invalid skills

Integration with Red Hat Trusted Artifact Signer (RHTAS)

• Fulcio for short-lived certificates (keyless signing)
• Rekor for transparency log
• TUF for root of trust distribution
• Compatible with enterprise Sigstore deployments

Implementation plan

Phase 1: Core signing and verification

• Add sigstore-go / cosign dependencies
• Implement pkg/verify/ — verify signatures on skill images
• Add skillctl verify command
• Add skillctl sign command (key-pair mode)

Phase 2: Keyless and policy

• Keyless signing via Fulcio/OIDC
• --verify flag on skillctl pull and skillctl install
• Configurable verification policy (require, warn, skip)
• Rekor transparency log integration

Phase 3: Enterprise (RHTAS)

• RHTAS endpoint configuration
• Custom TUF root support
• Integration with promote lifecycle (auto-sign on publish)

References

• Existing design: docs/design/2026-04-15-oci-skill-registry-design.md (lines 757–961)
• Implementation spec: docs/design/2026-04-16-implementation-spec.md (deferred to phase 2+)
• Sigstore
• Red Hat Trusted Artifact Signer

Start by reading docs/design/2026-04-15-oci-skill-registry-design.md at lines 757–961 and the deferred implementation spec, then inspect go.mod and the existing skillctl command entry points. The issue spans core signing, verification, keyless policy, and enterprise RHTAS phases; done requires the selected phase's commands, dependencies, and verification behavior to match the stated design.