feat: serve agent findings from backend

[breman27]

Summary

• Add a backend plugin surface for typed agent snapshots and prioritized findings.
• Move fleet and health derivation out of the browser into backend core logic.
• Update the fleet page and AgentInfoCard to consume the backend API.
• Rename the backend package to @showcall/backstage-plugin-agent-catalog-backend, which now provides both catalog ingestion and the HTTP plugin.
• Wire the renamed package and API into the demo, docs, release workflow, and changesets.
• Add ADR 0011 describing the backend findings/provenance boundary.

AgentInfoCard

The card now uses typed AgentSnapshot data and explicitly handles loading, missing snapshots, and backend failures so an API outage cannot look like agent deletion.

Validation

• Frontend package build passed
• Backend package build passed
• Frontend tests passed: 13 tests
• Backend tests passed: 111 tests
• AgentInfoCard focused suite passed: 8 tests
• git diff --check passed
• Frontend lint exits successfully with three existing no-mixed-plugin-imports warnings from the frontend type imports in this refactor