feat: support custom tools in subagents

All three SDKs (Go, Python, Node) use per-session tool handler lookup keyed
to the exact session ID. When the CLI creates child sessions for subagents,
those child session IDs are never registered in the SDK's sessions map.
Tool calls arriving with a child session ID fail with "unknown session".

This PR adds the ability to call custom tools for sub-agents in Go,
following the proposal in #947

Author: NGUYEN Duc Trung

go/README.md

@@ -359,6 +359,58 @@ safeLookup := copilot.DefineTool("safe_lookup", "A read-only lookup that needs n
 safeLookup.SkipPermission = true
 ```
 
+### Custom Tools with Subagents
+
+When a session is configured with both custom tools and custom agents (subagents), the
+subagents can invoke the parent session's custom tools. The SDK automatically routes
+tool calls from child sessions back to the parent session's tool handlers.
+
+#### Tool Access Control
+
+The `Tools` field on `CustomAgentConfig` controls which custom tools each subagent can access:
+
+| `Tools` value | Behavior |
+|---------------|----------|
+| `nil` (default) | Subagent can access **all** custom tools registered on the parent session |
+| `[]string{}` (empty) | Subagent cannot access **any** custom tools |
+| `[]string{"tool_a", "tool_b"}` | Subagent can only access the listed tools |
+
+#### Example
+
+```go
+session, err := client.CreateSession(ctx, &copilot.SessionConfig{
+    Tools: []copilot.Tool{
+        copilot.DefineTool("save_output", "Saves output to storage",
+            func(params SaveParams, inv copilot.ToolInvocation) (string, error) {
+                // Handle tool call — works for both direct and subagent invocations
+                return saveToStorage(params.Content)
+            }),
+        copilot.DefineTool("get_data", "Retrieves data from storage",
+            func(params GetParams, inv copilot.ToolInvocation) (string, error) {
+                return getData(params.Key)
+            }),
+    },
+    CustomAgents: []copilot.CustomAgentConfig{
+        {
+            Name:        "researcher",
+            Description: "Researches topics and saves findings",
+            Tools:       []string{"save_output"}, // Can only use save_output, not get_data
+            Prompt:      "You are a research assistant. Save your findings using save_output.",
+        },
+        {
+            Name:        "analyst",
+            Description: "Analyzes data from storage",
+            Tools:       nil, // Can access ALL custom tools
+            Prompt:      "You are a data analyst.",
+        },
+    },
+})
+```
+
+When `researcher` is invoked as a subagent, it can call `save_output` but not `get_data`.
+When `analyst` is invoked, it can call both tools. If a subagent attempts to use a tool
+not in its allowlist, the SDK returns a `"Tool '{name}' is not supported by this client instance."` response to the LLM.
+
 ## Streaming
 
 Enable streaming to receive assistant response chunks as they're generated: