fix(spawn): resolve parent agent from context in managed mode

[xthanhn91]

Problem

In managed mode, NewSpawnTool and NewSubagentTool are constructed with a hardcoded parentID of "default" (gateway.go:246-247). When a non-default agent spawns a subagent, the announce routes back through gateway_consumer.go using parent_agent metadata — which is "default". The session key resolves to agent:default:..., and since agent "default" does not exist in managed mode, the announce fails:

level=INFO  msg="subagent spawned" parent=default ...
level=INFO  msg="subagent announce → scheduler" session=agent:default:rua-telegram:group:...
level=ERROR msg="subagent announce: agent run failed" error="agent default not found: agent not found: default"

This affects all non-default agents in managed mode deployments.

Fix

Add ctxAgentKey to tool context keys (same pattern as ctxChannel, ctxChatID, etc.):

1. context_keys.go — new ctxAgentKey constant + WithToolAgentKey / ToolAgentKeyFromCtx
2. loop.go — inject l.id (agent key) into ctx before tool execution
3. subagent_spawn_tool.go — read agent key from ctx, fall back to construction-time parentID
4. subagent_tool.go — same ctx-based resolution for RunSync

Standalone mode is unaffected: when ctxAgentKey is empty, the fallback "default" is used.

Test plan

• go build ./... passes
• Managed mode: secondary agent spawns subagent → announce routes to correct parent session
• Standalone mode: spawn still works (fallback to "default")