F-mcp-screenshot — expose the window screenshot as an MCP tool (pixel companion to snapshot)

[bastien-gallay]

Summary

Expose termherd's window screenshot (the F-capture rung-1 PNG, #108) as an MCP
screenshot tool, so a driving Claude session can see the rendered UI — the
pixel companion to the text snapshot (#212). Child of #90.

Why it is its own slice

Unlike snapshot/list_sessions, a screenshot is not a pure read of
core::App state. The pixels come from an async iced window::screenshot
Task (see Shell::perform_capture), so answering the tool needs a different
bridge shape than the timeout-bounded state read the current bridge provides:
the shell must trigger the window screenshot, await it, encode a PNG, and return
bytes. That async round-trip (and how MCP carries the image content) is the real
work here, kept out of #212 so the snapshot slice stayed a clean state read.

Scope

• A bridge path that requests a window screenshot and returns encoded PNG
  bytes (or a saved path), bounded by a timeout like the state read.
• screenshot MCP tool returning image content.
• Best-effort: a window-less/headless run degrades gracefully (no panic),
  the text snapshot staying the reliable path.

Depends on

#212 (perception surface) and #193 (live bridge). Reuses the F-capture PNG
pipeline (#108).