feat: enforce Hermes skill and provider contracts

[offendingcommit]

Summary

• distribute the migrate-agent-plugin skill as a consumable Hermes plugin-kit skill
• strictly validate SKILL.md frontmatter, including supported metadata.hermes fields, at declaration and registration time
• expose the Hermes subagent lifecycle through a checked host accessor
• support memory, image-generation, and video-generation provider registration while retaining Hermes provider ABC instances
• document the contracts and verify them against a fresh Hermes Agent checkout

Validation

• HERMES_AGENT_PATH="$(pwd)/.hermes-agent" make test (97 tests)
• make test-contract (Hermes Agent 49c632310dd6877302e8dfa92e740b0ceddb97b8)
• make build
• git diff --check