feat: add guest network autoconfiguration via Firecracker MMDS

Introduces optional per-clone IP assignment using the Firecracker Microvm
Metadata Service (MMDS). A background daemon (fc-net-init) is baked into
the rootfs during init and captured in the golden snapshot — on clone
resume it polls 169.254.169.254 and applies the IP/GW/DNS config injected
by the orchestrator immediately after snapshot restore.

- config.go: add AutoNetConfig bool (FC_AUTO_NET_CONFIG=1)
- orchestrator.go: embed fc-net-init daemon + MMDS link-local route in
  init script; set AllowMMDS: true on golden NIC; spawnOne/SpawnSingle
  accept net bool and propagate it via FC_AUTO_NET_CONFIG in proxy env
- console.go: set AllowMMDS: true on clone NIC; call configureMmds()
  after m.Start() when AutoNetConfig is enabled
- network.go: add configureMmds() — PUT /mmds with ip/gw/dns over the
  clone's Firecracker Unix socket
- serve.go: POST /clones accepts optional {"net": bool} body to override
  the global AutoNetConfig default per-request
- web/terminal.html: spawn button always sends {"net": true}
- docs/commands.md: document manual config + MMDS autoconfiguration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

orchestrator/web/terminal.html

@@ -200,7 +200,11 @@
       spawnBtn.disabled = true;
       spawnBtn.textContent = 'Spawning…';
       clearError();
-      fetch('/clones', { method: 'POST' })
+      fetch('/clones', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ net: true }),
+      })
         .then(r => {
           if (!r.ok) return r.text().then(t => { throw new Error(t); });
           return r.json();