Feature request: Allow Copilot inline suggestions to use instruction files

Copilot inline suggestions should support a dedicated, opt-in instruction file for ghost text completions.

I am not asking for all existing chat or agent instruction files to be applied automatically to inline suggestions. That would likely add too much unrelated context and could hurt completion quality or latency.

Instead, there should be a separate instruction surface specifically for inline suggestions, for example:

* a dedicated workspace file for inline completions
* or a dedicated inline-suggestions section in existing customization files
* or a setting that explicitly opts selected instruction files into inline completions

Use case:
I work with a proprietary language for which the Copilot suggestions are not trained. This results in suggesting wrong operators, non-existing language features and functions.

Today, even if the workspace contains instructions that explain these rules, Copilot inline suggestions still propose JavaScript-like code in those embedded script fields (the proprietary language is vaguely similar to JavaScript). Chat can be guided by instructions and works well, but ghost text inline suggestions cannot.

Requested behavior:

* support an instruction file or configuration specifically for inline suggestions
* make it opt-in, not automatic for all instruction files
* allow teams to provide short, completion-focused rules for custom DSLs, embedded languages, or project-specific conventions
* scope it to the current file/language when possible

Why this would help:

* improves inline suggestions for custom DSLs and embedded languages
* avoids forcing all chat-oriented instructions into the inline completion prompt
* gives teams a focused way to steer ghost text behavior without overloading it
* reduces plausible-looking but invalid suggestions in domain-specific code

This request is specifically about ghost text / inline suggestions, not chat, inline chat, or BYOK/custom models.