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/connection.test.ts

@@ -489,15 +489,17 @@ describe('runKtxConnection', () => {
   it('rejects unknown drivers with a helpful error', async () => {
     const projectDir = join(tempDir, 'project');
     await initKtxProject({ projectDir });
-    await writeConnections(projectDir, {
-      mystery: { driver: 'duckdb' },
-    });
+    await writeFile(
+      join(projectDir, 'ktx.yaml'),
+      'connections:\n  mystery:\n    driver: duckdb\n',
+      'utf-8',
+    );
     const io = makeIo();
 
     await expect(
       runKtxConnection({ command: 'test', projectDir, connectionId: 'mystery' }, io.io),
     ).resolves.toBe(1);
-    expect(io.stderr()).toContain('uses driver "duckdb"');
-    expect(io.stderr()).toContain('Supported:');
+    expect(io.stderr()).toContain('connections.mystery.driver');
+    expect(io.stderr()).toContain('postgres');
   });
 });