fix(cold-storage): add configurable max storage limit

[patil2001]

Summary

Add MAX_COLD_STORAGE_GB variable to cap cold storage usage.
When enabled, archived models exceeding the limit are pruned
oldest-first. Cold storage (llm-cold-storage/) grows unboundedly
as more models are archived, with no eviction mechanism today.
This prevents hundreds of GB from accumulating silently on
backup drives.

AI Assistance

AI-assisted; reviewed and verified locally before opening.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Dashboard API / host agent
• Installer / bootstrap / lifecycle
• Docker Compose / service manifests
• Model routing / Hermes / capabilities
• Network exposure / auth / proxy
• Dependencies / runtime wiring

Risk And Validation

• Risk level: Medium
• Validation run:
  • git diff --check
  • Not required because: narrow change; syntax and diff checks pass locally

Commands/results:

git diff --check clean; bash -n on touched scripts passes