feat(message): add external flag to ToolPart to exclude plugin tool calls from model context

External tool parts are displayed in the TUI and persisted, but excluded
from toModelMessages() to prevent orphaned tool_use/tool_result pairs.
Plugins and HTTP endpoints can mark their tool executions as external.

Author: micuintus

packages/opencode/src/session/message-v2.ts

@@ -344,6 +344,10 @@ export namespace MessageV2 {
     tool: z.string(),
     state: ToolState,
     metadata: z.record(z.string(), z.any()).optional(),
+    /** When true, this tool execution was initiated externally (not by the LLM).
+     *  External parts are displayed in the TUI and persisted, but excluded from
+     *  model messages to prevent orphaned tool_use/tool_result pairs. */
+    external: z.boolean().optional(),
   }).meta({
     ref: "ToolPart",
   })
@@ -719,6 +723,9 @@ export namespace MessageV2 {
               type: "step-start",
             })
           if (part.type === "tool") {
+            // External tool executions (from plugins/HTTP, not LLM) are display-only.
+            // Including them would create orphaned tool_use/tool_result pairs.
+            if (part.external) continue
             toolNames.add(part.tool)
             if (part.state.status === "completed") {
               const outputText = part.state.time.compacted ? "[Old tool result content cleared]" : part.state.output