Add minimal control plane with REST API, database, and reconciler

- REST API (chi router): sandbox CRUD, exec, pause/resume, file write/read
- PostgreSQL persistence via pgx/v5 + sqlc (sandboxes table with goose migration)
- Connect RPC client to host agent for all VM operations
- Reconciler syncs host agent state with DB every 30s (detects TTL-reaped sandboxes)
- OpenAPI 3.1 spec served at /openapi.yaml, Swagger UI at /docs
- Added WriteFile/ReadFile RPCs to hostagent proto and implementations
- File upload via multipart form, download via JSON body POST
- sandbox_id propagated from control plane to host agent on create

internal/db/models.go

@@ -0,0 +1,26 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.30.0
+
+package db
+
+import (
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+type Sandbox struct {
+	ID           string             `json:"id"`
+	OwnerID      string             `json:"owner_id"`
+	HostID       string             `json:"host_id"`
+	Template     string             `json:"template"`
+	Status       string             `json:"status"`
+	Vcpus        int32              `json:"vcpus"`
+	MemoryMb     int32              `json:"memory_mb"`
+	TimeoutSec   int32              `json:"timeout_sec"`
+	GuestIp      string             `json:"guest_ip"`
+	HostIp       string             `json:"host_ip"`
+	CreatedAt    pgtype.Timestamptz `json:"created_at"`
+	StartedAt    pgtype.Timestamptz `json:"started_at"`
+	LastActiveAt pgtype.Timestamptz `json:"last_active_at"`
+	LastUpdated  pgtype.Timestamptz `json:"last_updated"`
+}