feat(rules-injector): add GitHub Copilot instructions format support (#403)

* feat(rules-injector): add GitHub Copilot instructions format support

- Add .github/instructions/ directory to rule discovery paths
- Add applyTo as alias for globs field in frontmatter parser
- Support .github/copilot-instructions.md single-file format (always-apply)
- Filter .github/instructions/ to only accept *.instructions.md files
- Add comprehensive tests for parser and finder

Closes #397

* fix(rules-injector): use cross-platform path separator in calculateDistance

path.relative() returns OS-native separators (backslashes on Windows),
but the code was splitting by forward slash only, causing incorrect
distance calculations on Windows.

Use regex /[/\]/ to handle both separator types.

---------

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/hooks/rules-injector/constants.ts

@@ -14,10 +14,17 @@ export const PROJECT_MARKERS = [
 ];
 
 export const PROJECT_RULE_SUBDIRS: [string, string][] = [
+  [".github", "instructions"],
   [".cursor", "rules"],
   [".claude", "rules"],
 ];
 
+export const PROJECT_RULE_FILES: string[] = [
+  ".github/copilot-instructions.md",
+];
+
+export const GITHUB_INSTRUCTIONS_PATTERN = /\.instructions\.md$/;
+
 export const USER_RULE_DIR = ".claude/rules";
 
 export const RULE_EXTENSIONS = [".md", ".mdc"];