Unify CLI UX and add PR description updates in fix loop

[chanakyav]

Summary

This PR unifies the CLI user experience and adds agent-driven PR description updates.

1. Unified post-start experience (Closes #5)

• Problem: autopilot start and autopilot fix-ci had different output formats after launching a session.
• Fix: Extract _launch_in_tmux() shared helper that all commands (start, fix-ci, resume, restart) use. Every launch now prints a standardized summary:

  ✓ Autopilot session started
    Task:     a1b2c3d4
    Mode:     review
    Branch:   autopilot/a1b2c3d4
    tmux:     autopilot-a1b2c3d4
  
    autopilot status              — check progress
    autopilot logs --session ...  — view logs
    tmux attach -t ...            — attach to session
    autopilot stop ...            — stop task
  

• Removes ~60 lines of duplicated tmux launch + output code from 4 command handlers.

2. Update PR description throughout the loop (Closes #2)

• Problem: PR description was only generated during the initial IMPLEMENT phase and never updated during fix iterations, even when the approach changed.
• Fix: Agent-driven approach — fix_prompt() now includes step 8 instructing the agent to update the PR title/description via gh pr edit if the implementation approach changed significantly during fixes.
• This keeps the orchestrator simple (no new state or API calls needed). The agent uses its judgment to decide when an update is warranted.

Tests

• 93 tests passing (no regressions)