Add agent_not_found and invalid_agent_address error codes

docs/protocol-error-status-model.md

@@ -46,6 +46,23 @@ Fields:
 - `at` (required, RFC3339 timestamp)
 - `details` (optional key-value map)
 
+### Standard Error Codes
+
+The `code` field is a stable, machine-readable identifier. The following codes are defined by the protocol:
+
+| Code | Category | Retryable | Description |
+|---|---|---|---|
+| `agent_not_found` | `transport` | No | The target agent does not exist in the registry. |
+| `invalid_agent_address` | `validation` | No | The agent address is syntactically invalid or does not conform to the expected format. |
+| `schema_invalid` | `validation` | No | The intent payload does not conform to the expected schema. |
+| `auth_failed` | `auth` | No | Authentication or authorization failed. |
+| `policy_denied` | `policy` | No | The request was rejected by a policy rule. |
+| `delivery_failed` | `transport` | Yes | The intent could not be delivered to the target agent. |
+| `intent_timeout` | `timeout` | Yes | The intent exceeded its deadline without completion. |
+| `internal_error` | `internal` | Yes | An unexpected internal error occurred. |
+
+Implementations may define additional codes within the `^[a-z0-9_:.\-]{3,128}$` pattern. Standard codes should be used where applicable for cross-implementation interoperability.
+
 Usage:
 
 - Used for predictable API and workflow error payloads.

schemas/protocol/intent.error.v1.json

@@ -14,7 +14,17 @@
   "properties": {
     "code": {
       "type": "string",
-      "pattern": "^[a-z0-9_:.\\-]{3,128}$"
+      "pattern": "^[a-z0-9_:.\\-]{3,128}$",
+      "examples": [
+        "agent_not_found",
+        "invalid_agent_address",
+        "schema_invalid",
+        "auth_failed",
+        "policy_denied",
+        "delivery_failed",
+        "intent_timeout",
+        "internal_error"
+      ]
     },
     "message": {
       "type": "string",