Removes hardcoded _MISSION_SIGNALS and _MISSION_DEFAULTS dicts from
generate_cover_letter.py. Domains and signals are now defined in
config/mission_domains.yaml, which ships with the original 5 domains
(music, animal_welfare, education, social_impact, health) plus 3 new
ones (privacy, accessibility, open_source).
Any key in user.yaml mission_preferences not present in the YAML is
treated as a user-defined domain with no signal detection — custom
note only. Closes#78.
