Remove semantic workspace search integration

[jdneo]

Remove Eclipse watched-file indexing and project-agent completion while preserving workspace-scoped customizations.

Reason for this decision

1. Based on the benchmark, semantic search does not provide significant accurate improvement or lower token consunption
2. There are so many workaround logic to support it in our code base, making code hard to understand
3. It will impact on the startup perf due to the local index.

[Copilot]

Pull request overview

This PR removes the Eclipse-side “semantic workspace search” integration (watched-file indexing + project/workspace agent completion plumbing) while keeping workspace-folder–scoped customization discovery (skills/prompts/instructions) via workspace folders.

Changes:

• Deletes the watched-files indexing pipeline (client request/response types, manager, deltas → CLS notifications) and forces watchedFiles=false in initialization capabilities.
• Removes the “workspace context” preference UI/defaults/feature-flag plumbing and related restart prompt behavior.
• Removes @... agent completion handling (including @workspace → @project rewrite) and updates/adds unit tests accordingly.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CONTEXT.md	Documents “workspace folder” vs semantic indexing, and “workspace instructions” semantics.
com.microsoft.copilot.eclipse.ui/.../preferences/messages.properties	Removes strings for the retired workspace-context preference + restart prompt.
com.microsoft.copilot.eclipse.ui/.../preferences/Messages.java	Removes NLS fields for the deleted preference/restart strings.
com.microsoft.copilot.eclipse.ui/.../preferences/CopilotPreferenceInitializer.java	Drops default initialization of the removed workspace-context preference.
com.microsoft.copilot.eclipse.ui/.../preferences/ChatPreferencesPage.java	Removes workspace-context toggle UI and restart-on-change logic.
com.microsoft.copilot.eclipse.ui/.../chat/services/ChatCompletionService.java	Removes agent command plumbing and resource-change listener; keeps template refresh via customization-file events.
com.microsoft.copilot.eclipse.ui/.../chat/ChatView.java	Stops rewriting @workspace to @project; persists/sends the raw message.
com.microsoft.copilot.eclipse.ui/.../chat/ChatAssistProcessor.java	Removes @-triggered completion activation and agent proposal generation.
com.microsoft.copilot.eclipse.ui.test/.../chat/services/ChatCompletionServiceTest.java	Updates mocks and asserts @workspace/@project are not treated as commands.
com.microsoft.copilot.eclipse.ui.test/.../chat/ChatAssistProcessorTest.java	Adds coverage ensuring auto-activation characters are slash-only.
com.microsoft.copilot.eclipse.core/.../utils/PlatformUtils.java	Removes workspace-root URI helper previously used by watched-files/agent plumbing.
com.microsoft.copilot.eclipse.core/.../lsp/WatchedFileManager.java	Deleted: removes workspace watched-file scanning + change tracking implementation.
com.microsoft.copilot.eclipse.core/.../lsp/protocol/GetWatchedFilesResponse.java	Deleted: removes watched-files protocol type.
com.microsoft.copilot.eclipse.core/.../lsp/protocol/GetWatchedFilesRequest.java	Deleted: removes watched-files protocol type.
com.microsoft.copilot.eclipse.core/.../lsp/protocol/DidChangeCopilotWatchedFilesParams.java	Deleted: removes watched-files delta protocol type.
com.microsoft.copilot.eclipse.core/.../lsp/protocol/CopilotCapabilities.java	Keeps watchedFiles capability in payload but hard-codes it to false; updates ctor signature.
com.microsoft.copilot.eclipse.core/.../lsp/protocol/ConversationAgent.java	Deleted: removes client-side agent model used for completion.
com.microsoft.copilot.eclipse.core/.../lsp/LsStreamConnectionProvider.java	Stops consulting workspace-context flag when building init options; always advertises watchedFiles disabled.
com.microsoft.copilot.eclipse.core/.../lsp/CopilotLanguageServerConnection.java	Removes agent listing + watched-files notifications/progress plumbing; removes workspace-root folder injection.
com.microsoft.copilot.eclipse.core/.../lsp/CopilotLanguageServer.java	Removes the conversation/agents request surface.
com.microsoft.copilot.eclipse.core/.../lsp/CopilotLanguageClient.java	Removes workspaceFolders() override and the copilot/watchedFiles request handler.
com.microsoft.copilot.eclipse.core/.../FeatureFlags.java	Removes isWorkspaceContextEnabled() legacy preference reader.
com.microsoft.copilot.eclipse.core/.../Constants.java	Removes WORKSPACE_CONTEXT_ENABLED constant.
com.microsoft.copilot.eclipse.core/META-INF/MANIFEST.MF	Drops core-bundle dependency on JGit (no longer needed after removing gitignore parsing in watched-files).
com.microsoft.copilot.eclipse.core.test/.../lsp/WatchedFileManagerTests.java	Deleted: removes tests tied to the removed watched-files manager.
com.microsoft.copilot.eclipse.core.test/.../lsp/LsStreamConnectionProviderTests.java	Adds test ensuring legacy workspace-context preference no longer affects watchedFiles capability.