refactor(rules-injector): split finder.ts into rule discovery modules

Extract rule finding logic:
- project-root-finder.ts: project root detection
- rule-file-finder.ts: rule file discovery
- rule-file-scanner.ts: filesystem scanning for rules
- rule-distance.ts: rule-to-file distance calculation

src/hooks/rules-injector/rule-file-finder.ts

@@ -0,0 +1,119 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, join } from "node:path";
+import {
+  PROJECT_RULE_FILES,
+  PROJECT_RULE_SUBDIRS,
+  USER_RULE_DIR,
+} from "./constants";
+import type { RuleFileCandidate } from "./types";
+import { findRuleFilesRecursive, safeRealpathSync } from "./rule-file-scanner";
+
+/**
+ * Find all rule files for a given context.
+ * Searches from currentFile upward to projectRoot for rule directories,
+ * then user-level directory (~/.claude/rules).
+ *
+ * IMPORTANT: This searches EVERY directory from file to project root.
+ * Not just the project root itself.
+ *
+ * @param projectRoot - Project root path (or null if outside any project)
+ * @param homeDir - User home directory
+ * @param currentFile - Current file being edited (for distance calculation)
+ * @returns Array of rule file candidates sorted by distance
+ */
+export function findRuleFiles(
+  projectRoot: string | null,
+  homeDir: string,
+  currentFile: string,
+): RuleFileCandidate[] {
+  const candidates: RuleFileCandidate[] = [];
+  const seenRealPaths = new Set<string>();
+
+  // Search from current file's directory up to project root
+  let currentDir = dirname(currentFile);
+  let distance = 0;
+
+  while (true) {
+    // Search rule directories in current directory
+    for (const [parent, subdir] of PROJECT_RULE_SUBDIRS) {
+      const ruleDir = join(currentDir, parent, subdir);
+      const files: string[] = [];
+      findRuleFilesRecursive(ruleDir, files);
+
+      for (const filePath of files) {
+        const realPath = safeRealpathSync(filePath);
+        if (seenRealPaths.has(realPath)) continue;
+        seenRealPaths.add(realPath);
+
+        candidates.push({
+          path: filePath,
+          realPath,
+          isGlobal: false,
+          distance,
+        });
+      }
+    }
+
+    // Stop at project root or filesystem root
+    if (projectRoot && currentDir === projectRoot) break;
+    const parentDir = dirname(currentDir);
+    if (parentDir === currentDir) break;
+    currentDir = parentDir;
+    distance++;
+  }
+
+  // Check for single-file rules at project root (e.g., .github/copilot-instructions.md)
+  if (projectRoot) {
+    for (const ruleFile of PROJECT_RULE_FILES) {
+      const filePath = join(projectRoot, ruleFile);
+      if (existsSync(filePath)) {
+        try {
+          const stat = statSync(filePath);
+          if (stat.isFile()) {
+            const realPath = safeRealpathSync(filePath);
+            if (!seenRealPaths.has(realPath)) {
+              seenRealPaths.add(realPath);
+              candidates.push({
+                path: filePath,
+                realPath,
+                isGlobal: false,
+                distance: 0,
+                isSingleFile: true,
+              });
+            }
+          }
+        } catch {
+          // Skip if file can't be read
+        }
+      }
+    }
+  }
+
+  // Search user-level rule directory (~/.claude/rules)
+  const userRuleDir = join(homeDir, USER_RULE_DIR);
+  const userFiles: string[] = [];
+  findRuleFilesRecursive(userRuleDir, userFiles);
+
+  for (const filePath of userFiles) {
+    const realPath = safeRealpathSync(filePath);
+    if (seenRealPaths.has(realPath)) continue;
+    seenRealPaths.add(realPath);
+
+    candidates.push({
+      path: filePath,
+      realPath,
+      isGlobal: true,
+      distance: 9999, // Global rules always have max distance
+    });
+  }
+
+  // Sort by distance (closest first, then global rules last)
+  candidates.sort((a, b) => {
+    if (a.isGlobal !== b.isGlobal) {
+      return a.isGlobal ? 1 : -1;
+    }
+    return a.distance - b.distance;
+  });
+
+  return candidates;
+}