schemas: add x_initiator and billing fields to token-usage schema

Document the new optional fields added to token-usage.jsonl records:
- x_initiator: the X-Initiator header value sent (agent/user)
- billing: parsed X-Quota-Snapshot-* and X-RateLimit-* response headers

These fields enable offline analysis of PRU billing behavior.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox

schemas/token-usage.schema.json

@@ -94,6 +94,45 @@
       "type": "integer",
       "minimum": 0,
       "description": "Total number of bytes in the response body (optional, omitted for WebSocket upgrades)."
+    },
+    "x_initiator": {
+      "type": "string",
+      "enum": ["agent", "user"],
+      "description": "The X-Initiator header value sent on the request. Determines billing treatment: 'agent' requests are not billed as premium, 'user' requests are. Present only for Copilot-bound requests."
+    },
+    "billing": {
+      "type": "object",
+      "description": "Billing/quota information extracted from upstream response headers (X-Quota-Snapshot-*, X-RateLimit-*). Present only when the upstream returns billing headers.",
+      "properties": {
+        "quota_chat": {
+          "type": "object",
+          "description": "X-Quota-Snapshot-Chat parsed fields.",
+          "properties": {
+            "ent": { "type": "string", "description": "Entitlement count (-1 = unlimited)." },
+            "ov": { "type": "string", "description": "Overage requests made this period." },
+            "ovPerm": { "type": "string", "description": "Whether overage is allowed (true/false)." },
+            "rem": { "type": "string", "description": "Percentage of entitlement remaining." },
+            "rst": { "type": "string", "description": "Quota reset date (RFC 3339 UTC)." }
+          },
+          "additionalProperties": true
+        },
+        "quota_premium-chat": {
+          "type": "object",
+          "description": "X-Quota-Snapshot-Premium-Chat parsed fields. Tracks premium request units (PRU).",
+          "properties": {
+            "ent": { "type": "string", "description": "Premium entitlement count." },
+            "ov": { "type": "string", "description": "Premium overage requests made." },
+            "ovPerm": { "type": "string", "description": "Whether premium overage is allowed." },
+            "rem": { "type": "string", "description": "Percentage of premium entitlement remaining." },
+            "rst": { "type": "string", "description": "Premium quota reset date." }
+          },
+          "additionalProperties": true
+        },
+        "rate_limit": { "type": "string", "description": "X-RateLimit-Limit value." },
+        "rate_remaining": { "type": "string", "description": "X-RateLimit-Remaining value." },
+        "rate_reset": { "type": "string", "description": "X-RateLimit-Reset value (Unix timestamp)." }
+      },
+      "additionalProperties": true
     }
   }
 }