fix: allocate fresh network on start VM

[sjmiller609]

Note

Allocates a fresh network and regenerates the config disk when starting a VM, adds cleanup rollback, and introduces a pre-build clean step in CI with an expanded Makefile clean target.

• Backend (instances):
  • lib/instances/start.go:
    • Allocate fresh network via CreateAllocation; update stored IP/MAC.
    • Add gVisor cleanup stack to rollback network on failures.
    • Regenerate config disk before boot with new network config.
    • Simplify flow; remove recreate/get allocation logic.
• CI:
  • .github/workflows/test.yml: Add make clean step before build.
• Build:
  • Makefile: Add clean to phony targets; expand clean to remove embedded Cloud Hypervisor and Caddy binaries and exec-agent.

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