feat: add watch command with configurable session reminders

[e6a5]

Adds flow watch - a long-running process that provides gentle reminders to help users remember to start, pause, resume, or end flow sessions.

Features

• Configurable reminders via ~/.config/flow/config.yml
• Smart state detection - tracks session transitions automatically
• Non-intrusive timestamps - designed for a dedicated terminal tab

Reminder Types

• Start sessions after idle time (default: 15m)
• Take breaks during long sessions (default: 2h)
• Resume paused sessions (default: 5m)

Usage

# Terminal 1: Start watching
flow watch

# Terminal 2: Work normally  
flow start "deep work"
flow end
# → Watch automatically reminds you to start again after 15 minutes

Configuration

watch:
  interval: "5m"              # Check frequency
  remind_after_idle: "15m"    # Idle reminder timing
  remind_after_pause: "5m"    # Pause reminder timing  
  remind_after_active: "2h"   # Break suggestion timing

Solves: Users forgetting to manage flow sessions
Breaking Changes: None - fully backward compatible