release: prepare public 0.5.0 curation

[kwalus]

Summary

• prepare the public 0.5.0 release candidate, including the first public Canopy Module and source_layout documentation surface
• bring the public repo forward with the current channels, feed, messages, bookmarks, module runtime, tray, MCP, networking, and security improvements
• refresh the public-facing docs and onboarding so the README, API reference, agent onboarding, MCP quickstart, peer connect guide, quickstart, and tray docs all describe the current product surface

Test plan

• GitHub Actions public Python checks pass on 3.10, 3.11, and 3.12
• GitHub Actions public web asset checks pass
• spot-check the README and core docs for public-facing clarity and consistency with the shipped runtime