VCL garbage collection: set cold state and limit versions

[jensens]

Context

Architecture §3.4 defines VCL lifecycle management after push:

1. After vcl.use <new>: set old VCL to cold state via vcl.state <old> cold
2. Wait 5-10 seconds before discarding (worker-thread references)
3. Maintain max 3 VCL versions (configurable)

Current state

Agent PushVCL (admin.go:163-195):

• Loads and activates new VCL ✓
• Discards old VCL after 5s sleep ✓
• Does NOT set old VCL to cold state first ✗
• Does NOT enforce max VCL count ✗
• Does NOT query vcl.list to check existing versions ✗

Risk

Without vcl.state cold, rapid VCL pushes can leave worker threads referencing old VCL. Without max version enforcement, VCL versions accumulate on endpoint churn.

Implementation

In internal/agent/admin.go PushVCL:

1. After vcl.use <new>, call vcl.state <old> cold
2. Before discard, query vcl.list and discard all but the 3 most recent
3. Add vinyl_vcl_versions_loaded Prometheus gauge