Implement leave recording command

[pigsly]

1. Add conf/leaves.json to store entries { "MM/DD": hours }.

2. Create a new script leave.py that:

   • Prompts for date and leave hours (multiple of 0.5).
   • Writes/updates the entry in conf/leaves.json.

3. Modify migrate.py to read conf/leaves.json and subtract leave hours from the day’s max_work_hours.

4. When generating a schedule for a leave day, append a row to the CSV with project ID P00000000, task Special Leave, and the leave hours.

5. Include leave.py in setup.py executables.

6. Document the leave command usage in both READMEs.