Add public A2A Agent Card endpoint that exposes registered agents as skills
for cross-platform agent discovery. Includes admin CLI for updating agent
capabilities, which populate skill tags and descriptions in the card.
- internal/a2a: new package with AgentCard generator, HTTP handler, and tests
- agents store/service: add ListAllActiveAgents (excludes human accounts)
- admin socket: add agent.update_capabilities command
- admin CLI: add `agent update-capabilities --name --capabilities` subcommand
- main.go: register /.well-known/agent-card.json route (public, no auth)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
