[cherry-pick] agentHost: Make Agent Merge enablement experiment configurable

[vs-code-engineering]

Cherry-pick of #334973 from release/1.137.

Summary

• allow experiments to override the default value of chat.agentMerge.enabled
• use automatic experiment updates so the resolved value continues to sync through the existing Agent Host configuration bridge

Testing

• npm run eslint -- src/vs/platform/agentHost/common/agentHostStarter.config.contribution.ts
• npm run typecheck-client

[Copilot]

🟢 Approval recommended

The change is minimal, consistent with existing experiment: { mode: 'auto' } usage in the same configuration contribution, and aligns with the stated goal of enabling automatic experiment updates.

Pull request overview

Makes the chat.agentMerge.enabled setting eligible for automatic experiment-driven overrides so the effective value can be updated dynamically and propagated through the existing Agent Host configuration bridge.

Changes:

• Adds experiment: { mode: 'auto' } to chat.agentMerge.enabled configuration schema entry.
• Enables experimentation service to override the default without requiring manual refresh/restart paths.

File summaries

File	Description
src/vs/platform/agentHost/common/agentHostStarter.config.contribution.ts	Marks chat.agentMerge.enabled as auto-experimentable so experiments can override the default and keep syncing to the agent host.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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