Delegate sub-agent enablement to language server

[jdneo]

resolve #304

[Copilot]

Pull request overview

This PR delegates sub-agent enablement entirely to the Copilot language server by removing Eclipse-side gating (preferences/policy DTO/event plumbing) and always advertising the subAgent capability during LSP initialization, aligning the client with server-authoritative policy enforcement.

Changes:

• Add an ADR documenting why sub-agent enablement is server-governed and remove overlapping client-side mechanisms.
• Remove the sub-agent toggle and related UI preference plumbing, messages, and constants.
• Stop deserializing/propagating sub-agent policy in policy/didChange, and always advertise sub-agent capability at init.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/adr/0001-sub-agent-enablement-is-server-governed.md	Adds an ADR capturing the rationale/decision to make sub-agent enablement server-authoritative.
CONTEXT.md	Introduces glossary terms clarifying “sub-agent”, “sub-agent policy”, and capability semantics.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/messages.properties	Removes UI strings for the sub-agent preference toggle.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/Messages.java	Removes the corresponding NLS fields for deleted sub-agent preference strings.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/CopilotPreferenceInitializer.java	Drops the default value initialization for the removed sub-agent preference key.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/ChatPreferencesPage.java	Removes the sub-agent toggle UI and related preference-driven tool configuration updates.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/policy/DidChangePolicyParams.java	Removes the sub-agent policy field and updates equals/hashCode/toString accordingly.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/LsStreamConnectionProvider.java	Always advertises subAgent capability as enabled during initialization.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/CopilotLanguageClient.java	Stops reacting to sub-agent policy changes from policy/didChange.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/FeatureFlags.java	Removes sub-agent policy state and preference-bridging logic.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/events/CopilotEventConstants.java	Removes the sub-agent policy change event topic constant.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/Constants.java	Removes the SUB_AGENT_ENABLED preference key constant.

[ethanyhou]

LGTM