fix: PTY stdin error, double-click select with full buffer, alt+key passthrough

- Remove proc.stdin.on() call (Bun FileSink has no .on method), add
  try/catch and proc.killed guards to write/resize
- Double-click enters select mode instead of shift+click, with yellow
  banner and Esc-to-exit hint for discoverability
- Select mode now dumps full scrollback buffer (up to 5000 lines) so
  users can scroll up and copy old conversation text
- Pass all Alt+key combos through input parser to PTY (fixes
  Alt+Backspace word deletion and other Alt shortcuts)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/pty/capture.ts

@@ -195,6 +195,14 @@ class VtScreen {
     }
   }
 
+  // Get full buffer: all scrollback lines + current screen (for select mode)
+  getAllLines(): string[] {
+    const lines: string[] = []
+    for (const row of this.scrollback) lines.push(this.renderRow(row))
+    for (let r = 0; r < this.height; r++) lines.push(this.renderRow(this.cells[r]))
+    return lines
+  }
+
   // Get screen as lines with embedded ANSI SGR codes (like tmux capture-pane -e)
   // When scrollOffset > 0, shows scrollback history mixed with screen content
   getLines(): string[] {
@@ -613,6 +621,12 @@ export function getLatestFrame(sessionName: string): CaptureResult | null {
   }
 }
 
+export function getFullBuffer(sessionName: string): string[] | null {
+  const state = panes.get(sessionName)
+  if (!state) return null
+  return state.screen.getAllLines()
+}
+
 export function stopCapture(sessionName: string): void {
   const state = panes.get(sessionName)
   if (!state) return