feat(opencode): add subagent timeline support (#658)

Map OpenCode `task` tool events into the existing sub_agent timeline shape,
enabling live activity tracking for OpenCode subagent sessions.

- Add `childSessionId` (optional) to sub_agent schema
- Make `actions` optional with default [] for backward compatibility
- Parse task tool input/output into sub_agent detail (tool-call-detail-parser)
- Track child session linkage and buffer/flush child tool parts
- Render sub_agent actions and session IDs in the app component
- Add glob tool detail mapping
- Include backward-compat schema test

Co-authored-by: Ryan Swift <ryan@mlh.io>

packages/server/src/shared/messages.tool-call-schema.test.ts

@@ -147,6 +147,30 @@ describe("shared messages tool_call schema", () => {
     }
   });
 
+  it("defaults missing sub_agent actions for older payloads", () => {
+    const parsed = AgentTimelineItemPayloadSchema.parse({
+      type: "tool_call",
+      callId: "call_sub_agent_legacy",
+      name: "Task",
+      status: "running",
+      error: null,
+      detail: {
+        type: "sub_agent",
+        subAgentType: "Explore",
+        description: "Inspect repository structure",
+        log: "[Read] README.md",
+      },
+    });
+
+    expect(parsed.type).toBe("tool_call");
+    if (parsed.type === "tool_call") {
+      expect(parsed.detail.type).toBe("sub_agent");
+      if (parsed.detail.type === "sub_agent") {
+        expect(parsed.detail.actions).toEqual([]);
+      }
+    }
+  });
+
   it("parses plain_text detail with icon and rejects unknown icon names", () => {
     const parsed = AgentTimelineItemPayloadSchema.parse({
       type: "tool_call",

packages/server/src/shared/messages.ts

@@ -412,14 +412,18 @@ const ToolCallDetailPayloadSchema: z.ZodType<ToolCallDetail, z.ZodTypeDef, unkno
       type: z.literal("sub_agent"),
       subAgentType: z.string().optional(),
       description: z.string().optional(),
+      childSessionId: z.string().optional(),
       log: z.string(),
-      actions: z.array(
-        z.object({
-          index: z.number().int().positive(),
-          toolName: z.string(),
-          summary: z.string().optional(),
-        }),
-      ),
+      actions: z
+        .array(
+          z.object({
+            index: z.number().int().positive(),
+            toolName: z.string(),
+            summary: z.string().optional(),
+          }),
+        )
+        .optional()
+        .default([]),
     }),
     z.object({
       type: z.literal("plain_text"),