fix(mitm): inject x-omniroute-source and x-omniroute-agent headers in server.cjs intercept (C2, master plan §3.5)

`MitmHandlerBase.fetchRouter` already injects the AgentBridge correlation
headers, but the CJS proxy in `src/mitm/server.cjs` had never been
updated to match. As a result the running Antigravity flow was hitting
the OmniRoute router with no source/agent identification, breaking the
contract documented in master-plan-group-A.md §3.5 and §12 acceptance #17.

This commit adds:

- `x-omniroute-source: agent-bridge` — distinguishes AgentBridge traffic
  from other inbound clients.
- `x-omniroute-agent: <id>` — IDE agent id resolved from the Host header
  via the existing `TARGET_HOST_AGENT` map (populated by `targets.json`
  + the antigravity baseline). Defensive fallback to `"unknown"` for
  hosts that were never registered, so router-side filters never get
  an empty value.

Antigravity non-regression preserved: `daily-cloudcode-pa.googleapis.com`
continues to resolve to `agentId="antigravity"` via the baseline seed in
`TARGET_HOST_AGENT.set(h, "antigravity")` at the top of the file.

src/mitm/server.cjs

@@ -367,11 +367,21 @@ async function intercept(req, res, bodyBuffer, mappedModel) {
     const body = JSON.parse(bodyBuffer.toString());
     body.model = mappedModel;
 
+    // C2 — Inject AgentBridge correlation headers per master plan §3.5.
+    // The OmniRoute router uses these to distinguish AgentBridge traffic from
+    // other inbound clients and to record the originating IDE agent id.
+    // Resolve agent id from the Host header against the target map; defensive
+    // fallback to "unknown" when the host is somehow not in the map.
+    const reqHost = String(req.headers.host || "").split(":")[0].toLowerCase();
+    const agentId = TARGET_HOST_AGENT.get(reqHost) || "unknown";
+
     const response = await fetch(ROUTER_URL, {
       method: "POST",
       headers: {
         "Content-Type": "application/json",
         Authorization: `Bearer ${API_KEY}`,
+        "x-omniroute-source": "agent-bridge",
+        "x-omniroute-agent": agentId,
       },
       body: JSON.stringify(body),
     });