CLI command support for custom skills, prompts, and agents paths.

### Describe the feature or problem you'd like to solve

I’d like Copilot CLI to support custom search paths for skills, prompts, and agents through CLI commands. Right now, managing these locations in terminal workflows is not straightforward, which makes setup less convenient and harder to keep consistent across projects and machines. A built-in path management flow in CLI would make customization easier, faster, and more predictable for everyday use.

### Proposed solution

Add a simple CLI command group for managing custom paths for skills, prompts, and agents.

Suggested commands:

copilot env paths list
copilot env paths add --type --path --scope
copilot env paths remove --type --path --scope
copilot env paths reset --type --scope
Optional:

copilot env paths sync-vscode
This improves convenience by:

Reducing manual setup steps
Making path configuration discoverable and scriptable
Keeping local and repo-specific customization easy to manage
Providing clear diagnostics when paths are missing or unreadable

### Example prompts or workflows

Example prompts or workflows

Quick local setup
Command: copilot env paths add --type skills --path ./copilot/skills --scope repo
Outcome: custom repo skills are available immediately.
Personal shared prompt folder
Command: copilot env paths add --type prompts --path ~/copilot/prompts --scope user
Outcome: favorite prompts are available across repos without repeated setup.
Reusable agent collection
Command: copilot env paths add --type agents --path ~/.copilot/agents --scope user
Outcome: custom agents are consistently available in CLI sessions.
Fast troubleshooting
Command: copilot env paths list
Outcome: user can quickly verify active paths, order, and scope.
Cleanup/reset
Command: copilot env paths reset --type prompts --scope user
Outcome: reset to a clean baseline when paths become outdated.

### Additional context

This request is primarily about convenience and consistency across IDE and terminal workflows.

Suggested acceptance criteria:

Commands exist to list/add/remove/reset paths by type and scope.
Configuration persists and is honored after restart.
Path precedence is deterministic and documented.
Invalid or inaccessible paths return actionable non-zero errors.
Effective path sources and order are visible in env diagnostics.
Non-goals for initial release:

Remote URL fetch or package registry integration.
Implicit trust elevation for untrusted path locations.

No files or tests are named in the issue. Start by locating the existing CLI command structure and current skills, prompts, and agents path configuration, then trace how user and repository settings are persisted and loaded. Done means the proposed commands, validation, persistence, precedence, diagnostics, and documented behavior are implemented.

索引モデルが issue の本文から書いたものです。