add chat.headers hook, adjust codex and copilot plugins to use it

packages/opencode/src/session/llm.ts

@@ -53,6 +53,7 @@ export namespace LLM {
       .tag("sessionID", input.sessionID)
       .tag("small", (input.small ?? false).toString())
       .tag("agent", input.agent.name)
+      .tag("mode", input.agent.mode)
     l.info("stream", {
       modelID: input.model.id,
       providerID: input.model.providerID,
@@ -131,6 +132,20 @@ export namespace LLM {
       },
     )
 
+    const { headers } = await Plugin.trigger(
+      "chat.headers",
+      {
+        sessionID: input.sessionID,
+        agent: input.agent,
+        model: input.model,
+        provider,
+        message: input.user,
+      },
+      {
+        headers: {},
+      },
+    )
+
     const maxOutputTokens = isCodex
       ? undefined
       : ProviderTransform.maxOutputTokens(
@@ -198,13 +213,6 @@ export namespace LLM {
       maxOutputTokens,
       abortSignal: input.abort,
       headers: {
-        ...(isCodex
-          ? {
-              originator: "opencode",
-              "User-Agent": `opencode/${Installation.VERSION} (${os.platform()} ${os.release()}; ${os.arch()})`,
-              session_id: input.sessionID,
-            }
-          : undefined),
         ...(input.model.providerID.startsWith("opencode")
           ? {
               "x-opencode-project": Instance.project.id,
@@ -218,6 +226,7 @@ export namespace LLM {
               }
             : undefined),
         ...input.model.headers,
+        ...headers,
       },
       maxRetries: input.retries ?? 0,
       messages: [