Subagent invocation fails with "unable to find agent" for agent that exists and is listed

## Bug
`InvokeSubagents` with `agent_name: "divedeep"` returns:
"unable to find agent with name: divedeep"

## Expected
Invocation should succeed since the agent is found by ListAgents and /agent swap.

## Steps
1. Agent configured at ~/.kiro/agents/divedeep.json
2. ListAgents → divedeep appears ✅
3. /agent swap divedeep → works ✅
4. InvokeSubagents with agent_name "divedeep" → fails ❌

## Config
- divedeep is in availableAgents and trustedAgents in the parent agent's toolsSettings
- Agent has custom MCP servers (builder-mcp, aws-coe, workplace-chat-mcp)
- Reproducible 100% of the time

Start by tracing the InvokeSubagents entry point and compare agent resolution with ListAgents and the /agent swap flow. Reproduce the configured divedeep case from ~/.kiro/agents/divedeep.json, including its availableAgents, trustedAgents, and custom MCP settings; done means InvokeSubagents can resolve and invoke the listed agent.

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