Extension: batch instructions should support nested strategy #58
Description
The batch instructions command (vscode-extension/src/commands/batch.ts) always calls generateCopilotInstructions() (flat generation), even when agentrc.config.json specifies strategy: "nested". It adjusts the output path to AGENTS.md but doesn't actually generate nested output (hub + detail files + optional CLAUDE.md).
Current behavior: Generates flat content, writes to AGENTS.md path when config says nested.
Expected: Use generateNestedInstructions() + writeNestedInstructions() when strategy is nested.
Complexity: Nested generation involves multi-file writes and interactive choices (detailDir, claudeMd) that are harder to batch. May need to read these from config rather than prompting per-root.