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

package.json

@@ -44,6 +44,7 @@
     "prepublishOnly": "npm run verify",
     "publish:npm": "npm publish --access public",
     "prepare": "node scripts/install-git-hooks.mjs",
+    "postinstall": "node scripts/postinstall.mjs",
     "changeset": "changeset",
     "release:version": "changeset version",
     "changelog:status": "changeset status"