feat(token-usage): enable token_usage_metrics by default in release builds

The v2 token-usage pipeline (transcript extraction, 5-minute bucket
events, authoritative pricing) has been validated behind the
token_usage_metrics feature flag with debug-only default. Flip the
release default to on so all environments collect token usage. The
flag remains overridable via GIT_AI_TOKEN_USAGE_METRICS / config, and
turning it off still deletes the token-usage database at daemon
startup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>