improve copilot AH customization discovery

[aeschli]

No description provided.

[Copilot]

Pull request overview

This PR updates the agent host’s Copilot customization discovery to better align with expected folder layouts, including support for discovering instruction files in nested directories and adding a user-home instructions root.

Changes:

• Adjust agent-folder discovery expectations around excluding README.md.
• Add discovery coverage for nested *.instructions.md files (workspace + user home).
• Update SessionCustomizationDiscovery search roots and scanning logic for instructions.

Show a summary per file

File	Description
src/vs/platform/agentHost/test/node/sessionCustomizationDiscovery.test.ts	Updates/expands discovery tests (README handling, nested instructions).
src/vs/platform/agentHost/node/copilot/sessionCustomizationDiscovery.ts	Extends discovery roots and adds recursive scanning for instruction files; adjusts agent scanning behavior.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/platform/agentHost/node/copilot/sessionCustomizationDiscovery.ts:166

• resolve(..., { resolveMetadata: true }) pulls size/mtime/etag metadata for every child entry, but the discovery scan only needs directory entries and names. This can add unnecessary IO overhead on large folders or remote file systems; consider using resolveMetadata: false (and IFileStat) here and in the recursive instruction traversal.

		let stat: IFileStatWithMetadata;
		try {
			stat = await this._fileService.resolve(rootUri, { resolveMetadata: true });
		} catch {
			// Root does not exist (or is unreadable) — nothing to discover or watch.
			return;
		}
		if (!stat.isDirectory || !stat.children) {
			return;

• Files reviewed: 2/2 changed files
• Comments generated: 1