observer: make @mentions clickable and open agent profile

[willwashburn]

Summary

• render known @agent mentions in MessageMarkdown as mention links with click callbacks
• style Observer mentions as bold blue links in message cards and thread rows
• wire mention clicks to select the agent and open the right-side Agent Profile pane
• ensure profile pane lookup can resolve offline/stale agents by searching all non-observer identities

Validation

• npm --prefix packages/react test -- src/tests/messageMarkdown.test.tsx
• npm --prefix packages/react run build
• npm --prefix packages/observer-dashboard run build

[github-actions]

Preview deployed!

Environment	URL
API	https://pr66-api.relaycast.dev
Health	https://pr66-api.relaycast.dev/health
Observer	https://pr66-observer.relaycast.dev

This preview shares the staging database and will be cleaned up when the PR is merged or closed.

Run E2E tests

npm run e2e -- https://pr66-api.relaycast.dev --ci

Open observer dashboard

https://pr66-observer.relaycast.dev