fix(agent): advertise inline image display by surface

[wgqqqqq]

Summary

• advertise inline Markdown image rendering only for Desktop/Web output surfaces
• avoid exposing unsupported renderer guidance to CLI, ACP, bots, and headless sessions
• route workspace image reads through explicit base64 encoding and remote connection context
• key local image caches by remote connection and path to prevent cross-host collisions

Verification

• pnpm --dir src/web-ui run test:run src/component-library/components/Markdown/Markdown.test.tsx
• pnpm run type-check:web
• cargo test -p bitfun-agent-runtime --test prompt_contracts
• cargo test -p bitfun-agent-runtime output_surface
• cargo test -p bitfun-desktop remote_file_bytes
• cargo check --workspace

Fixes #1538