feat(tui): run `!` shell commands interactively (PTY) so prompts can be answered

What feature would you like to see?

Run foreground ! shell commands interactively — under a PTY, with the user's keystrokes forwarded to the running command — so interactive commands can be answered.

Motivating example: !gh auth login prompts (Where do you use GitHub?, (Y/n)-style confirmations), but there is no way to answer. The same applies to ssh, sudo, npm init, scripts using read -p, etc. Today the only option is to leave the TUI and run these in a separate terminal.

Why it can't work today: ! commands reuse the agent's non-interactive Bash tool. The process is spawned with piped stdio and no TTY, and the RPC surface only exposes runShellCommand / cancelShellCommand — there is no path to forward user input to the running process. Note that a stdin pipe alone is not sufficient: gh auth login requires a TTY for its prompts (it hangs even with stdin redirected), so the motivating cases need a real PTY.

Proposal:

• Spawn foreground ! commands with a PTY (node-pty is already a dependency of the CLI and agent-core).
• While a ! command is running, route editor keystrokes to its stdin; keep Esc / Ctrl+C as cancel.
• Stream and record output in the transcript as today (shell_command context message).

Deliberately out of scope: a persistent / stateful shell session (the !! mode proposed in #2213). This request is only about answering the prompts of a single foreground command.

Additional information

• Related issues: #2213 (closed as not planned — a much broader persistent-shell request; this is the narrow slice), #2915 (raw escape sequences from foreground Bash output — PTY output makes control sequences unavoidable, so transcript rendering needs a decision: sanitize vs. interpret).
• A stdin-only variant (forward keystrokes, no PTY) would cover simple line-based prompts (read -p) but not TTY-checking tools like gh / ssh / sudo; PTY seems required for the motivating cases.
• Open questions for maintainers: PTY vs stdin-only; how the transcript should render control sequences; whether the recorded context should capture the raw stream or cleaned text.

Trace the existing runShellCommand and cancelShellCommand RPC paths and the TUI editor keystroke handling; node-pty is already listed as a CLI and agent-core dependency. Resolve the PTY and transcript-rendering questions, then verify that a foreground ! command accepts input, preserves Esc/Ctrl+C cancellation, and records its output.