Replace the revoke functionality with delete to properly clean up agent references.
When an agent is deleted:
- All direct device assignments are removed
- Agent is removed from site defaults
- Agent is removed from organization defaults
- Devices automatically fall back to site/org agents or cloud polling
This ensures no orphaned references and provides better clarity about
what happens to device monitoring when an agent is removed.
- Add delete_agent_token/1 function in Agents context
- Update LiveView to use delete_agent event instead of revoke_agent
- Update UI button from "Revoke" to "Delete" with clearer confirmation message
- Add comprehensive tests for delete functionality and fallback behavior
- All 792 tests passing, no Credo warnings