Update gtfs-sqljs skill for v0.6.0 pluggable adapter API

Rewrite SKILL.md to reflect the v0.6 surface: pluggable
GtfsDatabaseAdapter, createSqlJsAdapter / createBetterSqlite3Adapter /
wrapBetterSqlite3, GtfsSqlJs.attach(), async query methods + close +
export, ExportNotSupportedError for file-backed drivers. Drop removed
SQL/locateFile options from the options table and fix the Calendar
section (getCalendarByServiceId, not the non-existent getCalendars).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: chtitux