feat: Implement Copillot CLI's Plan mode

[DonJayamanne]

For https://github.com/github/copilot-agent-runtime/issues/5660

[Copilot]

Pull request overview

Adds an opt-in setting to enable AI-generated branch names when Copilot CLI creates isolated worktrees, wiring the generator into the Copilot CLI session participants (both the “controller” and V1 implementations).

Changes:

• Add new configuration key github.copilot.chat.cli.aiGenerateBranchNames.enabled (default: false) with localization.
• Conditionally instantiate GitBranchNameGenerator based on the new setting and pass it into Copilot CLI session participants.
• Thread an (optional) generated branch name into worktree initialization for new untitled CLI sessions; update unit tests for the new constructor parameter.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/platform/configuration/common/configurationService.ts	Registers the new config key in ConfigKey.Advanced.
src/extension/chatSessions/vscode-node/chatSessions.ts	Reads the new config and conditionally creates/passes GitBranchNameGenerator.
src/extension/chatSessions/vscode-node/copilotCLIChatSessions.ts	Makes branch name generator optional and guards its usage.
src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts	Adds optional branch-name generation and passes it into folder/worktree initialization for V1 participant.
src/extension/chatSessions/vscode-node/test/copilotCLIChatSessionParticipant.spec.ts	Updates participant construction to account for the new optional constructor parameter.
package.json	Contributes the new setting to VS Code configuration.
package.nls.json	Adds the localized description string for the new setting.

Comments suppressed due to low confidence (2)

src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts:1314

• There are two statements on the same line (const branchNamePromise = ...; const [model, agent] = ...). This makes the control flow harder to read and is easy to miss during reviews/merges. Split these into separate lines/statements (and consider extracting isNewSession and branchNamePromise into clearly named variables).

			};
			const branchNamePromise = (isUntitled && request.prompt && this.branchNameGenerator) ? this.branchNameGenerator.generateBranchName(fakeContext, token) : Promise.resolve(undefined); const [model, agent] = await Promise.all([
				this.getModelId(request, token),
				this.getAgent(id, request, token),

src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts:1313

• New behavior: when starting an untitled session, the participant may call branchNameGenerator.generateBranchName(...) and thread the result into initializeFolderRepository(... newBranch: ...). The existing unit tests for this participant never pass a GitBranchNameGenerator (they always pass undefined), so this feature path isn’t covered. Add a test that injects a stub generator and asserts it is invoked only for new sessions and that the resolved branch name is forwarded to worktree creation (via initializeFolderRepository/createWorktree).

			const requestTurn = new ChatRequestTurn2(request.prompt ?? '', request.command, [], '', [], [], undefined, undefined, undefined);
			const fakeContext: vscode.ChatContext = {
				history: [requestTurn],
				yieldRequested: false,
			};
			const branchNamePromise = (isUntitled && request.prompt && this.branchNameGenerator) ? this.branchNameGenerator.generateBranchName(fakeContext, token) : Promise.resolve(undefined); const [model, agent] = await Promise.all([
				this.getModelId(request, token),