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.

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

src/hooks/rules-injector/parser.ts

@@ -60,7 +60,7 @@ function parseYamlContent(yamlContent: string): RuleMetadata {
       metadata.description = parseStringValue(rawValue);
     } else if (key === "alwaysApply") {
       metadata.alwaysApply = rawValue === "true";
-    } else if (key === "globs" || key === "paths") {
+    } else if (key === "globs" || key === "paths" || key === "applyTo") {
       const { value, consumed } = parseArrayOrStringValue(rawValue, lines, i);
       // Merge paths into globs (Claude Code compatibility)
       if (key === "paths") {