Add agent management API endpoints

[kanghengliu]

Parent: #247

Goal

Expose REST endpoints for agent CRUD/test/search so every feature currently buried in cli/agent_mgmt.sh + cli/agent_mgmt.py is remotely accessible.

Tasks

• Inventory register/list/get/update/delete/toggle/test/test-all/search flows and their required payloads.
• Implement authenticated API routes with pagination + validation and avoid depending on .oauth-tokens/ingress.json on disk.
• Backfill automated tests (unit + integration) for each operation.
• Update CLI/docs to point to the HTTP API; keep optional thin wrappers for backwards compatibility.