fix(installer): read config.toml on re-run so user_name (and other user-scoped answers) are preserved as defaults (#2411)

loadExistingConfig only read from legacy _bmad/<module>/config.yaml files, but
the installer writes user-scoped answers (user_name, communication_language, etc.)
to _bmad/config.user.toml. On every subsequent reinstall those values were not
loaded back, so the user got re-prompted instead of seeing their prior answers as
defaults.

Adds parseCentralToml — a lightweight line scanner matching the installer's own
TOML output format — and updates loadExistingConfig to read config.toml and
config.user.toml first (merging both into the same section buckets). Legacy
per-module config.yaml files are kept as a fallback for pre-v6 installations.

Co-authored-by: RobertOcsko <robert.ocsko@;seon.io>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Brian <bmadcode@gmail.com>