feat(billing): GCP BigQuery billing-export cost source

[Cro22]

What

Adds a billing.Source that reads real net cost (list cost + credits) from the standard GCP billing export in BigQuery, grouped by service — the GCP counterpart to the existing AWS Cost Explorer source.

Selectable via CLOUDORACLE_BILLING_PROVIDER=gcp_bigquery; the v1 cost endpoints then report data_source: billing_gcp_bigquery.

How

• internal/billing/bigquery.go — BigQuerySource implements billing.Source. A narrow bigQueryAPI seam returns already-parsed (service, cost) rows so unit tests run fully offline (same flattening trick the GCP inventory listers use); the real client wraps *bigquery.Client via Application Default Credentials.
• Net cost SQL: SUM(cost + IFNULL((SELECT SUM(c.amount) FROM UNNEST(credits) c), 0)) grouped by service.description, filtered on the usage_start_time partition column.
• config: gcp_bigquery enum + CLOUDORACLE_GCP_BILLING_DATASET / CLOUDORACLE_GCP_BILLING_TABLE, with a cross-field check requiring project + dataset + table.
• main: billing wiring became a switch with the GCP case; loud fallback to the snapshots approximation on init failure (never refuses to start).
• Docs (configuration.md, cloud-providers.md), agent knowledge (data-sources-and-caveats.md), and the README roadmap updated.

Tests

internal/billing (sum/normalize, SQL bounds+table, error-wrap) and internal/config (cross-field required/OK) green; full go build ./... clean.

🤖 Generated with Claude Code