Allow github.copilot.chat.commitMessageGeneration.instructions to read files outside of the workspace

Allow github.copilot.chat.commitMessageGeneration.instructions and related instruction settings to read files outside the workspace

Currently, these Copilot instruction settings only accept { "file": "..." } entries that resolve inside the current workspace. That makes it impossible to reuse shared instruction files stored outside the repo, such as ~/.dotfiles/..., which is a useful setup for large, reusable prompts and specs.

There was already a PR opened for this in microsoft/vscode-copilot-release, but that repo is archived and issues there are no longer being actively worked on. I'm reopening the request here so it can be tracked in the current repository.

Please allow the following settings to load instruction files from outside the workspace as well:

• github.copilot.chat.commitMessageGeneration.instructions
• github.copilot.chat.reviewSelection.instructions
• github.copilot.chat.pullRequestDescriptionGeneration.instructions

Use case: I keep large shared prompt files and supporting specs in a personal directory and reference them from my personal user settings.

Expected behavior: Instruction file paths should be allowed to resolve from user settings or absolute paths, not just workspace-relative files.

Why this matters:

• Reusing shared prompt files avoids duplicating large instruction blocks across repos.
• It keeps settings.json clean and maintainable.
• It makes these Copilot features more practical for power users with standardized workflows.