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/shared/apiTypes.ts

@@ -297,6 +297,35 @@ export interface FileContentResponse {
   binary: boolean;
 }
 
+export interface WriteWorkspaceFileOptions {
+  createDirs?: boolean;     // default: true — mkdir -p equivalent
+  overwrite?: boolean;      // default: true — throw if false and file exists
+}
+
+export interface WriteWorkspaceFileResponse {
+  path: string;
+  size: number;
+  modifiedAt: string;
+  created: boolean;  // true if file was created, false if overwritten
+}
+
+export interface DeleteWorkspaceFileResponse {
+  path: string;
+  existed: boolean;  // true if file existed and was deleted, false if file did not exist
+}
+
+export interface MoveWorkspaceFileOptions {
+  createDirs?: boolean;   // default: true — mkdir -p equivalent for target parent directory
+  overwrite?: boolean;    // default: false — throw if target exists (safer default than writeFile)
+}
+
+export interface MoveWorkspaceFileResponse {
+  fromPath: string;
+  toPath: string;
+  size: number;
+  modifiedAt: string;
+}
+
 export type GitFileState = "unmodified" | "modified" | "added" | "deleted" | "renamed" | "copied" | "untracked" | "ignored" | "conflicted";
 
 export interface GitStatusFile {