feat: standardize data directory to ~/.codecora/trapfall/

[ajianaz]

Changes

• Default DB path: ~/.codecora/trapfall/trapfall.db (was: ./trapfall.db)
• Override via TRAPFALL_DATA_DIR env var
• Auto-create directory on first run
• TRAPFALL_DATABASE_URL and --db flag still take precedence

New functions

Config::default_data_dir()  // ~/.codecora/trapfall/
Config::default_db_path()   // ~/.codecora/trapfall/trapfall.db
Config::ensure_data_dir()   // mkdir -p if not exists

Tests

+3 new tests: default_data_dir, default_db_path, ensure_data_dir

ENV override priority

1. TRAPFALL_DATABASE_URL or --db (explicit)
2. TRAPFALL_DATA_DIR + trapfall.db (custom dir)
3. ~/.codecora/trapfall/trapfall.db (default)

Closes #230