Previously, removing a claude_code credential from the anthropic pool
only printed a note — the next load_pool() re-seeded it from
~/.claude/.credentials.json. Now writes a 'suppressed_sources' flag
to auth.json that _seed_from_singletons checks before seeding.
Follows the pattern of env: source removal (clears .env var) and
device_code removal (clears auth store state).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>