Extend `.agents` discovery to instructions, agents, and hooks for any opened folder

## Feature
Please extend .agents as a first-class customization root for Copilot in VS Code, both for workspace folders and for user-level storage, even when the opened folder is not a Git repository.

This request is specifically about the `.agents/` directory tree, not the root `AGENTS.md` file.

## Current state
Related to the open tracking issue microsoft/vscode#292410, whose checklist already includes "Support other primitives (agents, hooks, instructions/rules, plugins) in `.agents` folder." This issue proposes a concrete, scoped design for that checklist item, plus the additional non-Git-folder guarantee. See also microsoft/vscode#322506 for the related agent-host/session-discovery configurability angle.
- Skills already support `.agents/skills` and `~/.agents/skills`.
- Custom instructions currently cover `.github/copilot-instructions.md`, `*.instructions.md`, root `AGENTS.md`, and the `.claude/.copilot` instruction locations documented today, but not `.agents/instructions`.
- Custom agents currently use `.github/agents`, `.claude/agents`, and `~/.copilot/agents` by default; note that microsoft/vscode#322506 indicates the agent-host/session-discovery layer already hardcodes `.agents/agents` and `.agents/skills` as workspace-level search roots on the SDK/session side, so this request is to make `.agents` an official, documented default consistently across both surfaces, not just a configurable or partially hardcoded one.
- Hooks are Preview. Default locations already include `.github/hooks` and the existing hook settings locations; `.agents/hooks` is not among them.

## Proposed behavior
- Add `.agents/instructions/**/*.instructions.md` and `~/.agents/instructions/**/*.instructions.md` as discovered locations.
- Add `.agents/agents/` and `~/.agents/agents/` to the default agent-location set.
- Once hooks are stable enough for a default-path change, add `.agents/hooks/` and `~/.agents/hooks/` alongside the existing hook locations.
- Discovery should work for any opened workspace folder, including non-Git folders.

Note: `.agents/instructions`, `.agents/agents`, and `.agents/hooks` can already be added manually today via `chat.instructionsFilesLocations`, `chat.agentFilesLocations`, and `chat.hookFilesLocations`. This request is to add them to the default sets, not to add net-new capability.

## What a good solution would let me verify
- Opening a non-Git folder containing `.agents` (for example, `D:\work\MyProjectOutsideGit`) yields the same discovery behavior as a Git-backed folder.
- A workspace containing only `.agents/instructions`, `.agents/agents`, and (when hooks are no longer Preview) `.agents/hooks` behaves consistently without requiring `.github` / `.copilot` equivalents.
- Documented, deterministic handling for same-named files across same-scope roots where applicable.
- Existing `.github` / `.claude` / `.copilot` paths continue to work.

## Why this helps
- One portable customization root across VS Code and Copilot CLI.
- Less duplication across `.github`, `.claude`, and `.copilot`.
- Better fit for teams that standardize on `.agents` in plain folders and monorepos, not only Git repos.

## Related context
Skills already establish the `.agents/` precedent; this request is to close the gap for instructions, agents, and hooks. If needed, this can be split into separate issues per artifact type.