New remote file-download handlers and a prompt-paint layer for plugin UIs

What

Two new surfaces arrive together.

• File download and save handlers: one takes file_uuid, name and keep_existing and places the file in a session folder. It reports placed, kept, too_large or failed, and refuses providers that are not Anthropic's own with not_first_party ("not downloaded: not permitted on this API provider"). The other saves a file by file_id with save_as, bytes and session_folder, and returns codes such as no_room and name_taken. Both are guarded by CLAUDE_CODE_REMOTE or first-party provider checks, and a helper returns false unless CLAUDE_CODE_REMOTE is set. No caller was found for them in this change.
• Prompt paint: the list of plugin interface abilities (ui_attach) now includes ui_prompt_paint, next to ui_prompt_read, ui_prompt_fill and ui_prompt_suggest. The prompt interface gains paint and forget operations that draw layered text and decorations into the composer, the box where you type your message.

Why

Remote sessions look set to receive files directly into a session folder, though nothing calls these handlers yet. Plugins and remote surfaces can now draw over the prompt box, so text you see there may come from a plugin and not from you.