Create SCHEMA.md

SCHEMA.md

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+# CarbonOps Dataset Schema
+
+## 1) grid-intensity/
+CSV/Parquet with columns:
+- `timestamp` (UTC, ISO8601)
+- `region` (string, snake_case)
+- `intensity_gco2_per_kwh` (float, >=0)
+- `source` (enum: `uk_national_grid`, `entsoe`, `custom_csv`)
+- `source_url` (string, optional)
+- `updated_at` (UTC)
+
+Constraints:
+- `timestamp` monotonically increasing within region
+- one row per 30 min (GB) or configured resolution
+- missing data → `NaN`, never `-1`
+
+## 2) ci-samples/
+- `job_id` (string), `repo` (string), `duration_s` (float), `artifact_mb` (float), `runner_type` (enum), `timestamp_start` (UTC).