[codex] Add clipboard paste uploads

[firewalker06]

Summary

• Add Remote UI paste handling for clipboard-provided files/images in the agent composer and inquiry form.
• Synthesize safe filenames for nameless clipboard blobs before routing them through the existing pending attachment pipeline.
• Add Remote UI asset regression assertions for the clipboard attachment path.

Impact

Users can paste copied screenshots/images, and browser-exposed copied files, directly into the Remote UI composer instead of opening the file picker. Text-only paste continues to behave normally.

Validation

• bundle exec ruby test/remote_server_test.rb
• bin/test
• node --check lib/hq/remote_ui/assets/app.js
• Temporary Remote UI browser verification with Playwright + system Chrome: pasted PNG became a pending attachment, sent with the prompt, cleared from the composer, and rendered in the chat attachment row.