Document loops-serve control plane

[andrei-hasna]

Summary

• Document loops-serve in the README binary list, deployment-mode model, status commands, and install/runtime notes.
• Align docs/DEPLOYMENT_MODES.md and docs/USAGE.md so self-hosted docs describe loops-serve as the Postgres/RDS-backed HTTP control-plane host, while keeping local and cloud boundaries explicit.

Task

• Todos task: f867976d-69a4-4573-8746-ae199d8643db
• Source report: /home/hasna/.hasna/loops/reports/oss-repo-docs-diff-producer/20260706T232719Z/report.md

Validation

• repos docs drift /home/hasna/.hasna/loops/worktrees/open-loops/f867976d-69a4-4573-8746-ae199d8643db-d28580c9 --limit 20 --pretty -> status=ok, issues=0, missing_mentions=[]
• rg -n 'loops-serve' README.md docs -S -> hits README plus docs/DEPLOYMENT_MODES.md and docs/USAGE.md
• git diff --check -- README.md docs/DEPLOYMENT_MODES.md docs/USAGE.md -> clean
• bun run scripts/no-private-cloud-boundary.mjs -> OpenLoops private cloud boundary scan passed
• staged credential-pattern scan -> staged secrets scan passed

Review Notes

• Independent adversarial reviewer initially found stale linked deployment docs; this PR includes the follow-up doc alignment and the final reviewer pass reported no blockers.