feat: Plugin API Completeness — file mutations, prompt editor, and attachment APIs

- WorkspaceFiles: writeFile, deleteFile, moveFile with path safety
  - writeFile: text/binary, auto-create dirs, overwrite option
  - deleteFile: idempotent, uses lstat (removes symlinks not targets)
  - moveFile: unix mv semantics, overwrite defaults to false
  - All mutations auto-refreshFiles() in File Explorer
  - Symlink escape prevention via realpath(dirname) check

- PluginPromptEditor: insertText, getText, getSelection, onPaste, onKeyDown, focus
  - Uses CM6 EditorView.domEventHandlers() via Compartment (not raw DOM)
  - Handlers registered before mount are preserved and applied on mount
  - First-to-consume-wins ordering for multi-plugin scenarios
  - insertText replaces selection (not inserts after)

- PluginAttachments: insertFileReference, getAttachedFiles, removeFileReference
  - insertFileReference validates file exists before inserting @path
  - Does not auto-focus editor (unlike prompt.insertText)
  - @file regex requires file extension to avoid matching emails

- Server endpoints: PUT /file, DELETE /file, POST /file/move
  - All work for local and federated machines

- Tests: 31 unit tests, 9 integration tests, 5 client tests
- Docs: 3 new sections in plugins.md

src/server/workspaces/pathSafety.ts

@@ -30,11 +30,11 @@ export function normalizeRelativePath(input: string | undefined): string {
   return parts.join("/");
 }
 
-function isNodeErrorWithCode(error: unknown, code: string): error is NodeJS.ErrnoException {
+export function isNodeErrorWithCode(error: unknown, code: string): error is NodeJS.ErrnoException {
   return typeof error === "object" && error !== null && "code" in error && error.code === code;
 }
 
-function ensureInside(root: string, target: string): void {
+export function ensureInside(root: string, target: string): void {
   const rel = relative(root, target);
   if (rel === "") return;
   if (rel.startsWith("..") || isAbsolute(rel)) throw new Error("Path escapes workspace");