fix: discover AGENTS.md and CLAUDE.md at workspace roots

[joshspicer]

Companion to microsoft/vscode#307937.

The provider only discovered copilot-instructions.md via customInstructionsService.getAgentInstructions(). AGENTS.md and CLAUDE.md live at workspace roots and are also agent instructions but weren't found. Now stats workspace roots for these files and includes them as agent-instructions items.

This fixes the providerApi.enabled: true path where the Copilot CLI harness uses the provider to supply items.

[Copilot]

Pull request overview

Updates the Copilot CLI customization provider to also surface workspace-root agent instruction files (AGENTS.md, CLAUDE.md) alongside existing copilot-instructions.md, aligning provider-driven item discovery with expected agent-instructions behavior (notably for the providerApi.enabled: true Copilot CLI harness path).

Changes:

• Extend CopilotCLICustomizationProvider instruction discovery to stat workspace roots for AGENTS.md and CLAUDE.md and emit them as agent-instructions.
• Add/adjust unit tests to cover filesystem-based discovery of these workspace-root instruction files.

Show a summary per file

File	Description
src/extension/chatSessions/vscode-node/copilotCLICustomizationProvider.ts	Adds workspace-root discovery for AGENTS.md/CLAUDE.md and emits them under agent-instructions.
src/extension/chatSessions/vscode-node/test/copilotCLICustomizationProvider.spec.ts	Updates provider construction for new deps and adds a test validating root-file discovery via mocked filesystem.

Copilot's findings

Comments suppressed due to low confidence (1)

src/extension/chatSessions/vscode-node/test/copilotCLICustomizationProvider.spec.ts:387

• This new test also relies on inline as any mocks for IWorkspaceService and IFileSystemService. To keep tests maintainable and aligned with the codebase’s Vitest practices, prefer reusable mock classes (configurable for the “file exists” vs “not found” cases) rather than ad-hoc objects and any casts.

			const testProvider = disposables.add(new CopilotCLICustomizationProvider(
				mockPromptFileService,
				mockCopilotCLIAgents,
				mockCustomInstructionsService,
				mockPromptsService,
				new TestLogService(),
				{ getWorkspaceFolders: () => [workspaceRoot] } as any,
				{
					stat: (uri: URI) => existingUris.has(uri.toString())
						? Promise.resolve({ type: 1, ctime: 0, mtime: 0, size: 0 })
						: Promise.reject(new Error('not found')),
				} as any,
			));

• Files reviewed: 2/2 changed files
• Comments generated: 2