Add tool search to Copilot agent host

[bhavyaus]

Adds deferred tool search support to the Copilot agent host using the existing client-side semantic search.

Draft until VS Code bumps to @github/copilot-sdk@1.0.7 and @github/copilot@1.0.71.

[Copilot]

Pull request overview

Adds experimental deferred tool search to Copilot Agent Host, delegating semantic ranking to the Copilot extension. Dependency bumps remain pending.

Changes:

• Adds configuration, capability gating, prompt guidance, and SDK tool deferral.
• Bridges deferred tool metadata to client-side semantic search.
• Adds unit coverage for routing, metadata, models, and embeddings.

Show a summary per file

File	Description
agentHostCopilotCliSettingsContribution.test.ts	Tests setting forwarding.
chat.shared.contribution.ts	Registers the experimental setting.
agentHostSessionHandler.ts	Routes search calls to the client.
agentHostCopilotCliSettingsContribution.ts	Forwards tool-search configuration.
toolSearchDeferral.test.ts	Tests capability gating and constants.
copilotAgentSession.test.ts	Tests deferred-tool routing.
agentHostPromptRegistry.test.ts	Updates prompt test context.
agentMetaReaders.test.ts	Tests candidate metadata validation.
toolSearchDeferral.ts	Defines model support and exemptions.
toolInstructions.ts	Adds tool-search prompt guidance.
promptRegistry.ts	Applies guidance when search is active.
AGENTS.md	Documents deferred loading architecture.
copilotSessionLauncher.ts	Enables SDK tool search.
copilotAgentSession.ts	Implements the client search override.
toolSearchConstants.ts	Defines runtime/client tool names.
agentToolCallMeta.ts	Adds candidate metadata handling.
copilotCliConfig.ts	Defines root configuration.
toolSearchTool.ts	Searches an injected Agent Host corpus.
toolSearchTool.spec.ts	Tests injected and registry corpora.
toolEmbeddingsComputer.ts	Supports uncached dynamic embeddings.

Review details

• Files reviewed: 20/20 changed files
• Comments generated: 3
• Review effort level: Medium