Document native manifest and sub-agent sync

[Aaronontheweb]

Summary

• add SkillServer specs for skills, sub-agents, and native manifest sync
• document the native manifest and sub-agent sync epic with GitHub tracking links
• update README and project context to distinguish implemented RFC discovery from planned native manifest support

Verification

• git diff --check

Tracking

• Epic: Epic: Native manifest and sub-agent sync #84
• Sub-issues: Define native manifest and sub-agent sync specifications #85, Align skill artifact distribution with the Cloudflare RFC #86, Implement /manifest.json native manifest root and skill collection #87, Add sub-agent storage and artifact endpoints #88, Add sub-agent collection to the native manifest #89, Extend Netclaw.SkillClient for native manifest and sub-agents #90, Extend CLI validation and publishing for sub-agents #91, Add native manifest sync support in NetClaw #92, Harden security and compatibility for native sync #93