feat(debug): add close() method to DebugSession for resource lifecycle

[luojiyin1987]

Context

DebugSession currently has no explicit cleanup method. Adding close() establishes the lifecycle API needed for proper resource management in future PRs.

Scope

• src/output/debug.ts (~3 lines)
• Adds close: () => void to DebugSession type
• Both enabled and disabled sessions return a no-op close()

Benefits

• Establishes the lifecycle API for DebugSession
• Prepares for future PRs that will call debugSession.close() in finally/error paths
• No behavior change, pure infrastructure

Related

Part of the PR split plan for resource cleanup and exit code refactoring.