fix(agents): route spawn tool to target agent

[bogdanovich]

Summary

• pass the spawn tool's optional agent_id through to sub-turn execution
• run the child turn with the requested agent's model, tools, workspace, skills, and injected files
• return an explicit error when the requested target agent does not exist
• mark background spawn sub-turns as critical so they can continue after the parent turn finishes

Problem

The spawn tool already exposes an agent_id parameter and validates it with the configured allowlist, but the actual sub-turn execution ignored the target agent. The child turn was created from the parent/default agent instead, so spawn(agent_id: "...") could acknowledge the target while still running with the wrong agent configuration.

That breaks multi-agent setups where agents intentionally have different tools, workspaces, skills, or models.

Fix

This PR adds AgentID to the tool/agent sub-turn config path and resolves the target agent inside spawnSubTurn. If no agent_id is provided, behavior remains unchanged: the child turn uses the parent agent and falls back to the default agent.

Validation

go test ./pkg/tools ./pkg/agent -run 'TestSpawnTool|TestSubTurn|TestSpawn|SubTurn'
go test ./pkg/tools ./pkg/agent

[afjcjsbx]

Hello! Thanks for the PR. In the meantime, #2531 (#2531) was merged, your pull request now has merge conflicts. Could you please perform a rebase to resolve them? 🙏

[bogdanovich]

I updated this PR branch on top of the latest upstream/main and resolved the subturn conflict so the target-agent routing change applies cleanly again.

Validation rerun on the rebased branch:

• go test ./pkg/tools -run TestSpawn -count=1
• go test ./pkg/agent -run SubTurn -count=1

The branch should no longer be in a conflicting state.