refactor(plugin-api): trim plugin API scope to grounded capabilities

Builds on marcus's plugin-api-completeness work. Narrows the new plugin
surface to capabilities that expose real, otherwise-unreachable pi-web
functionality, and drops invented/duplicative surfaces:

Kept:
- files.writeFile / deleteFile / moveFile (genuine workspace mutation,
  federated, path-safe)
- prompt.insertText / getText / getSelection (editor state access)

Dropped:
- attachments.* (insertFileReference/getAttachedFiles/removeFileReference):
  getAttachedFiles invented a structured-attachment notion pi-web does not
  have and duplicated prompt.getText() + a regex with a false email-safety
  claim; insert/removeFileReference were thin sugar over readFile +
  insertText that plugins can compose themselves.
- prompt.onPaste / onKeyDown: an incomplete two-event hook system shaped
  around a single use case, overlapping the editor's native image-paste
  handling. Deferred until a real editor event/hook surface is designed.
- prompt.focus: redundant and buggier duplicate of the existing
  focusPrompt() (silently no-ops when not on the chat view). Focus stays
  as focusPrompt().

Security fix:
- deleteWorkspaceFile now resolves the parent via realpath + ensureInside
  before lstat/unlink, closing a symlinked-parent-directory escape that
  allowed deleting files outside the workspace (write/move already did
  this). Final path component is still not resolved, so deleting a symlink
  removes the link, not its target. Adds a regression test.

Docs and the registry test mock updated to match the trimmed surface.

.changeset/plugin-api-completeness.md

@@ -2,4 +2,4 @@
 "@jmfederico/pi-web": patch
 ---
 
-Add file mutation, prompt editor, and attachment APIs to the plugin system, completing the stable workspace interaction surface.
+Add workspace file mutation (`files.writeFile`, `files.deleteFile`, `files.moveFile`) and prompt editor (`prompt.insertText`, `prompt.getText`, `prompt.getSelection`) APIs to the plugin system. File mutations work for local and federated machines, enforce workspace path safety, and auto-refresh the File Explorer.