Enhance auto-injected copilot instructions to have more relevant instructions for scope

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

The applyTo glob pattern is not enough to scope down an instruction in large codebases with possibly hundreds of agent docs to read

### Proposed solution

Support domain/category "tags" as an addition to  applyTo  glob patterns in  .instructions.md  frontmatter, so instructions can be auto-injected based on a semantic tag assigned to files, in combination with or independently of path-based glob matching. For example, a repo currently maintaining a separate, hand-curated library of convention docs (because many conventions are cross-cutting and don't map to a clean glob) could instead express every one of those conventions as a plain  .instructions.md  file tagged by concern (e.g.  error-handling ,  security ) — removing the need for a parallel documentation system just to work around glob's path-only scoping.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by reading the .instructions.md frontmatter handling and the existing applyTo glob-matching behavior. Trace where files are assigned scope and where instructions are auto-injected; done should include semantic tags working independently of or alongside path matching, with coverage for the proposed error-handling and security examples.

Written by the indexing model from the issue text.