fix(drive): repair open runtime bugs

[kitlangton]

Why

Four independent runtime edges currently fail silently or misleadingly: chord-shaped press keys are typed as text, empty outer script errors hide useful nested causes, source-checkout programs lose the default-config asset during compilation, and screenshots omit terminal images.

What Changes

Before	After
ui.press("ctrl+u") types ctrl+u	fails immediately with UiPressError and directs callers to { ctrl: true }
Error("", { cause }) logs a blank line	prints the nested cause chain
relative source imports fail on default-config.jsonc	default config is bundled as TypeScript data
captured image planes are ignored	screenshots composite RGBA planes at their terminal-cell placement

UiPressError widens only the press operation. The canonical wire request remains { key, modifiers }; no chord alias or protocol convenience shape is added.

Image capture boundary

Drive accepts the optional CapturedFrame.images field introduced by OpenCode #49868. Older endpoints omit the field and retain existing text-only behavior; newer endpoints supply planes that Drive draws beneath its cursor and keypress overlays.

Scope

This closes #70, #71, and #77. It addresses the Drive half of #56; complete image evidence requires the companion OpenCode protocol PR.

Verification

cd packages/drive
bun run check
bun run test

• 277 Effect/unit tests pass.
• 62 CLI integration tests pass.
• Each bug has a regression test that failed before its corresponding implementation.
• The screenshot test decodes the generated PNG and verifies the captured image pixel.