refactor: inline schedule.py and fetch_team_mapping.py into release.py

Merge schedule parsing and team mapping directly into release.py,
reducing from 6 scripts to 4. Both were thin wrappers around gog CLI
calls + row parsing — no reason for separate files now that gog
handles the Google API complexity.

Shared gog helpers: _gog_sheets_get() and _gog_sheets_tabs().
_fetch_teams() now calls Google Sheets directly instead of spawning
fetch_team_mapping.py as a subprocess.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: durandom