mobile: paste/attach a screenshot or image into the agent

[pallaoro]

Summary

Let users paste/attach a screenshot or image into the agent from the phone — drop a mockup, an error screenshot, or a design into Claude Code (and other agents) directly from the mobile composer.

Why

This is the one mobile-UX feature — besides session persistence, now shipped in #70 — that shows up in the high-engagement "coding from the phone" discourse (e.g. @TermiusHQ, ~282 likes: paste mockups/error logs into Claude Code from iPhone/iPad). People want to hand the agent visual context without a laptop.

Current state

Not built. The composer explicitly defers it:

apps/mobile/src/Composer.tsx:5 — "Attachments are a fast-follow"

Why it's cheap — the plumbing already exists

This is a client-side feature, not new protocol work. The wire primitive is already there:

• util.writeImageBytes exists on the protocol side (@ateam/protocol) — the desktop already uses it to stage an image onto the box for the agent.
• The mobile client stubs the native slice (apps/mobile/src/connection.ts — stageImagePath/stageClipboardImage return false today) and routes file/image concerns over RPC.

So the work is: image picker (expo-image-picker) → read bytes → util.writeImageBytes to the box → reference the staged path in the prompt/keystrokes. No daemon or protocol changes expected.

Acceptance

• Composer has an "attach image" affordance (photo library + screenshot).
• Selected image is staged on the box via the existing RPC and made available to the running agent.
• Works for the claude agent at minimum.

Priority

Highest-value next build for closing the Termius-workflow gap, given the plumbing already exists. Note: may require expo-image-picker (native dep → prebuild).