Pitch

Companion to right-click copy. Add a keyboard shortcut (`Ctrl+Y`) that copies the currently-focused transcript cell to the system clipboard via the existing `arboard` integration. Picks up tool output, assistant prose, or a thinking block depending on what the cursor is on.

Optional: `Ctrl+Shift+Y` copies the cell PLUS its surrounding context (the tool call that produced this result + the assistant turn it belonged to) for sharing in bug reports.

Acceptance

• Ctrl+Y on a cell copies its plain-text content to system clipboard.
• Ctrl+Shift+Y copies cell + immediate context (parent turn).
• Status toast confirms "Copied to clipboard".
• Documented in /help.