Add query lint and check commands

og-cheet-sheet.md

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+# Omnigraph Cheat Sheet
+
+## Local Query Validation
+
+Use an explicit schema file:
+
+```bash
+omnigraph query lint --query ./queries.gq --schema ./schema.pg --json
+omnigraph query check --query ./queries.gq --schema ./schema.pg
+```
+
+Use a local or `s3://` repo target:
+
+```bash
+omnigraph query lint --query ./queries.gq ./repo.omni --json
+omnigraph query check --query ./queries.gq s3://bucket/repo
+```
+
+Use `omnigraph.yaml` target resolution:
+
+```bash
+omnigraph query lint --query ./queries.gq --target local --config ./omnigraph.yaml
+```
+
+## What It Checks
+
+- parses every query in the file
+- typechecks each query against the resolved schema
+- warns on hardcoded mutation queries with no params
+- warns when nullable node fields have no update-query coverage
+
+## Current Limits
+
+- repo-backed lint is local/S3-only in v1
+- HTTP targets need `--schema`
+- exit code is nonzero only when there are strict validation errors