fix(scan): unblock relationship discovery on Snowflake

Two adjacent bugs prevented the scan's relationship pipeline from producing
any joins on a Snowflake warehouse:

- relationship-profiling.ts fell through to a default `GROUP_CONCAT` branch
  for unknown drivers. Snowflake has no GROUP_CONCAT, so every per-table
  profile query failed with "Unknown function GROUP_CONCAT". Add an explicit
  Snowflake branch that uses LISTAGG with a literal '\x1f' delimiter
  (Snowflake requires the delimiter to be a constant, so CHR(31) is rejected).
- description-generation.ts destructured `connector.sampleTable` and
  `connector.sampleColumn` into bare locals, losing the `this` binding when
  the class-method connectors (Snowflake, Postgres, MySQL) were invoked.
  Every sample call threw "Cannot read properties of undefined (reading
  'assertConnection')" and degraded LLM descriptions to metadata-only
  prompts. Call the methods through the connector instead.

Without these, even after the primary-key probe is allowed to fail softly,
the scan ends up with 0 validated relationships and an empty `joins:` block
in every shard YAML.

packages/cli/src/context/scan/description-generation.ts

@@ -463,11 +463,11 @@ export class KtxDescriptionGenerator {
       }
     }
 
-    const sampleTable = input.connector.sampleTable;
+    const connector = input.connector;
     let sampleData: KtxTableSampleResult | null = null;
     let fallbackReason: 'capability_missing' | 'sampling_failed' | 'empty_sample' | null = null;
 
-    if (!sampleTable) {
+    if (!connector.sampleTable) {
       fallbackReason = 'capability_missing';
       this.logger?.warn('KTX scan connector does not support table sampling; falling back to metadata-only prompt', {
         connectorId: input.connector.id,
@@ -484,7 +484,7 @@ export class KtxDescriptionGenerator {
       try {
         sampleData = await retryAsync(
           () =>
-            sampleTable(
+            connector.sampleTable!(
               {
                 connectionId: input.connectionId,
                 table: tableRef,
@@ -684,11 +684,11 @@ export class KtxDescriptionGenerator {
           });
           columnValues = [];
         } else {
-          const sampleColumn = input.connector.sampleColumn;
+          const connector = input.connector;
           try {
             const sample = await retryAsync(
               () =>
-                sampleColumn(
+                connector.sampleColumn!(
                   {
                     connectionId: input.connectionId,
                     table: tableRef,

packages/cli/src/context/scan/relationship-profiling.ts

@@ -227,6 +227,9 @@ function sampleAggregateSql(driver: KtxConnectionDriver, innerSql: string): stri
   if (driver === 'clickhouse') {
     return `(SELECT arrayStringConcat(groupArray(toString(value)), '\\x1F') FROM (${innerSql}) AS relationship_profile_values)`;
   }
+  if (driver === 'snowflake') {
+    return `(SELECT LISTAGG(CAST(value AS VARCHAR), '\\x1f') FROM (${innerSql}) AS relationship_profile_values)`;
+  }
   return `(SELECT GROUP_CONCAT(CAST(value AS TEXT), char(31)) FROM (${innerSql}) AS relationship_profile_values)`;
 }