docs(agents): add release canary testing skill

[TaylorMutch]

Summary

Adds the test-release-canary agent skill and CI docs for manually dispatching, watching, and locally reproducing the Release Canary workflow. This is stacked on #1336 so the base PR stays focused on the Helm canary workflow change.

Related Issue

N/A - split out from #1336.

Changes

• Adds .agents/skills/test-release-canary/SKILL.md with manual dispatch, SHA-pinned chart testing, local kind reproduction, and failure triage guidance.
• Adds Release workflow documentation in CI.md with the canary trigger model and Helm chart version behavior.
• Adds test-release-canary to the contributor skill inventory.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable) - N/A, agent skill/docs only.