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.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"),