Add PTY-based end-to-end CLI testing infrastructure

Add infrastructure for testing interactive CLI features using pseudo-terminal (PTY) simulation.

Context

Currently, interactive prompts (jp_inquire, inquire) require a real TTY and cannot be tested automatically. While we can use dependency injection to test the logic around prompts, true end-to-end testing of the CLI's interactive behavior requires simulating terminal input/output.

Libraries like portable-pty or expectrl can create pseudo-terminals that allow injecting keystrokes and capturing output, enabling tests like:

• Interrupt handling (Ctrl+C menus)
• Tool permission prompts
• Multi-select dialogs
• Editor integration

Alternatives

Dependency injection at the prompt level (implemented separately) covers unit testing of prompt-adjacent logic but doesn't verify the actual terminal rendering or keystroke handling.

Proposed Implementation

1. Evaluate portable-pty vs expectrl for our needs
2. Create a test harness that spawns jp commands in a PTY
3. Add helper functions for sending keystrokes and matching output
4. Write e2e tests for critical interactive flows

Start by evaluating portable-pty and expectrl for spawning jp commands and simulating terminal input/output. Build a PTY test harness with helpers for keystrokes and output matching, then add end-to-end coverage for interrupt handling, permission prompts, multi-select dialogs, and editor integration.