feat(KB-104): move file browser from task modal to toolbar

- Add backend API routes and file service for project-scoped file operations
- Create useProjectFileBrowser and useProjectFileEditor hooks with comprehensive tests
- Update FileBrowserModal to support both task and project modes via dualMode prop
- Add Files button to dashboard header toolbar
- Remove Files tab from TaskDetailModal
- Integrate project file browser into main App layout
- Include changeset for @dustinbyrne/kb package

packages/dashboard/app/hooks/useProjectFileEditor.ts

@@ -0,0 +1,120 @@
+import { useState, useEffect, useCallback } from "react";
+import type { FileContentResponse, SaveFileResponse } from "../api";
+import { fetchProjectFileContent, saveProjectFileContent } from "../api";
+
+interface UseProjectFileEditorReturn {
+  content: string;
+  setContent: (content: string) => void;
+  originalContent: string;
+  loading: boolean;
+  saving: boolean;
+  error: string | null;
+  save: () => Promise<void>;
+  hasChanges: boolean;
+  mtime: string | null;
+}
+
+/**
+ * Hook for editing a file in the project directory.
+ *
+ * @param rootPath - The project root directory path (from config/store)
+ * @param filePath - The file path to edit (null if no file selected)
+ * @param enabled - Whether to enable loading (e.g., when editor is visible)
+ * @returns File editor state and controls
+ */
+export function useProjectFileEditor(
+  rootPath: string,
+  filePath: string | null,
+  enabled: boolean
+): UseProjectFileEditorReturn {
+  const [content, setContentState] = useState<string>("");
+  const [originalContent, setOriginalContent] = useState<string>("");
+  const [mtime, setMtime] = useState<string | null>(null);
+  const [loading, setLoading] = useState(false);
+  const [saving, setSaving] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const setContent = useCallback((newContent: string) => {
+    setContentState(newContent);
+    setError(null);
+  }, []);
+
+  // Load file content when filePath changes
+  useEffect(() => {
+    if (!enabled || !filePath) {
+      setContentState("");
+      setOriginalContent("");
+      setMtime(null);
+      setError(null);
+      return;
+    }
+
+    let cancelled = false;
+
+    async function loadFile() {
+      setLoading(true);
+      setError(null);
+
+      try {
+        const response: FileContentResponse = await fetchProjectFileContent(filePath!);
+
+        if (!cancelled) {
+          setContentState(response.content);
+          setOriginalContent(response.content);
+          setMtime(response.mtime);
+        }
+      } catch (err: any) {
+        if (!cancelled) {
+          setError(err.message || "Failed to load file");
+          setContentState("");
+          setOriginalContent("");
+          setMtime(null);
+        }
+      } finally {
+        if (!cancelled) {
+          setLoading(false);
+        }
+      }
+    }
+
+    loadFile();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [filePath, enabled]);
+
+  const hasChanges = content !== originalContent;
+
+  const save = useCallback(async () => {
+    if (!filePath || !hasChanges) {
+      return;
+    }
+
+    setSaving(true);
+    setError(null);
+
+    try {
+      const response: SaveFileResponse = await saveProjectFileContent(filePath, content);
+      setOriginalContent(content);
+      setMtime(response.mtime);
+    } catch (err: any) {
+      setError(err.message || "Failed to save file");
+      throw err;
+    } finally {
+      setSaving(false);
+    }
+  }, [filePath, content, hasChanges]);
+
+  return {
+    content,
+    setContent,
+    originalContent,
+    loading,
+    saving,
+    error,
+    save,
+    hasChanges,
+    mtime,
+  };
+}