feat(context): add driver-discriminated connection schemas (#96)

* refactor(context): export and describe mapping shape schemas

* feat(context): add driver-schemas module with warehouse drivers

* feat(context): add metabase, looker, lookml driver schemas with mappings

* feat(context): add notion, dbt, metricflow driver schemas

* refactor(context): make connectionSchema a driver-discriminated union

* chore(context): re-export KtxConnectionConfig from project package

* docs(context): add connection driver schema plan

* chore(secrets): allowlist example credentials in driver-schemas fixtures

* test(cli): align metabase fixtures with required api_url field

The driver-discriminated union added in this branch now requires api_url
for metabase connections and a known driver for warehouses. Update slow
CLI test fixtures and assertions so they exercise the new schema:
- ingest.test-utils.ts: add api_url to the prod-metabase fixture.
- setup.test.ts: switch metabase fixture from 'url' to 'api_url'.
- local-scan-connectors.test.ts: invalid-driver/missing-driver tests now
  expect the schema error from loadKtxProject (parse-time rejection).

packages/cli/src/setup.test.ts

@@ -311,7 +311,7 @@ describe('setup status', () => {
         '    url: env:DATABASE_URL',
         '  metabase:',
         '    driver: metabase',
-        '    url: env:METABASE_URL',
+        '    api_url: https://metabase.example.test',
         '    api_key_ref: env:METABASE_API_KEY',
         '    warehouse_connection_id: warehouse',
         'llm:',