feat: support custom tools in subagent sessions

Enable custom tools registered on a parent session to be dispatched when
called by subagent child sessions. The Copilot CLI creates separate child
sessions for subagents, and those sessions carry their own session IDs
that are not in the SDK's session registry.

Changes:

Go SDK (go/)
- Track child-to-parent session mappings via subagent.started events
- Implement resolveSession() to transparently route child session IDs
  back to the parent session that owns the tool handlers
- Enforce per-agent tool allowlists (CustomAgentConfig.Tools) for both
  Protocol v2 (tool.call RPC) and Protocol v3 (external_tool.requested
  broadcast events)
- Auto-populate ToolDefinitions on CustomAgentConfig in the session.create
  and session.resume wire payloads so the CLI can propagate custom tool
  metadata to child sessions
- Preserve child mappings after subagent.completed/failed for in-flight
  requests; clean up on parent session destroy or client stop

Design & Docs
- Add cross-SDK design spec (docs/design/subagent-tool-dispatch.md)
  covering protocol contract, data model, allowlist enforcement, tool
  advertisement, and language-specific implementation notes
- Document tool advertisement and v3 enforcement in go/README.md

Tests
- Unit tests for session resolution, tool allowlist enforcement,
  v3 broadcast deny/allow, enrichAgentToolDefinitions, and instance
  tracking (go/client_subagent_test.go)
- Integration test scaffolding for subagent custom tool invocation
  and denied-tool scenarios (go/internal/e2e/subagent_tool_test.go)
- Placeholder E2E snapshots (test/snapshots/subagent_tool/)

Author: NGUYEN Duc Trung

docs/design/subagent-tool-dispatch.md

@@ -368,6 +368,62 @@ 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.
+
+> **Tool advertisement:** When custom agents reference tools via the `Tools` allowlist, the SDK automatically includes full tool definitions (`ToolDefinitions`) in the agent configuration sent to the CLI. This enables the CLI to propagate custom tool metadata to child sessions.
+>
+> **Protocol v3 enforcement:** Protocol v3 broadcast tool events from child sessions are subject to the same allowlist enforcement as Protocol v2 RPC tool calls.
+
 ## Streaming
 
 Enable streaming to receive assistant response chunks as they're generated: