feat(greenplum): expose distribution key + storage type to chat agent

When connected to a Greenplum cluster, schema introspection now surfaces
the two attributes that dominate query performance there: the data
distribution policy (DISTRIBUTED BY / RANDOMLY) and the storage kind
(heap / AO row / AO column). Without these, the agent writes
PG-optimal SQL that triggers Redistribute Motion in GP and runs orders
of magnitude slower than necessary.

- track Greenplum major version (6 vs 7) at connect time, since GP6
  uses pg_class.relstorage and GP7 dropped it in favor of pg_am
- new fetch_gp_table_extras helper queries gp_distribution_policy and
  the version-appropriate storage catalog, returns a per-table map
- format_table_block prints `  -- GP: DISTRIBUTED BY (...) | STORAGE: ...`
  under the TABLE header when extras are available
- build_overview_postgres appends a GREENPLUM NOTES block with the
  distribution-aware-join rules and a skew-detection one-liner; the
  agent sees this in every system prompt on a GP connection
- build_schema_context (legacy generate_sql / explain_sql / fix_sql_error
  path) and the chat get_columns tool both feed extras into format_table_block

P0 of the GP support arc — partitioning, EXPLAIN motion-aware parsing,
external tables, resource queues, and a skew-check tool are deliberately
deferred to follow-up commits.

src-tauri/src/commands/chat_tools.rs

@@ -6,7 +6,8 @@
 
 use crate::commands::ai::{
     fetch_column_comments, fetch_columns, fetch_enum_types, fetch_foreign_keys_raw,
-    fetch_table_comments, fetch_unique_constraints, format_table_block, ColumnInfo,
+    fetch_gp_table_extras, fetch_table_comments, fetch_unique_constraints, format_table_block,
+    ColumnInfo,
 };
 use crate::commands::connections::{load_connection_config, switch_database_core};
 use crate::commands::saved_queries::{list_saved_queries_core, save_query_core};
@@ -219,6 +220,8 @@ async fn get_columns_postgres(
     requested: &[(String, String, String)],
 ) -> TuskResult<String> {
     let pool = state.get_pool(connection_id).await?;
+    let is_greenplum = matches!(state.get_flavor(connection_id).await, DbFlavor::Greenplum);
+    let gp_major = state.get_gp_major(connection_id).await.unwrap_or(7);
 
     let (col_res, fk_res, enum_res, tbl_comm_res, col_comm_res, unique_res) = tokio::join!(
         fetch_columns(&pool),
@@ -234,6 +237,11 @@ async fn get_columns_postgres(
     let tbl_comments = tbl_comm_res.unwrap_or_default();
     let col_comments = col_comm_res.unwrap_or_default();
     let uniques = unique_res.unwrap_or_default();
+    let gp_extras = if is_greenplum {
+        Some(fetch_gp_table_extras(&pool, gp_major).await)
+    } else {
+        None
+    };
 
     // Build (schema, table) → Vec<ColumnInfo>
     let mut by_table: BTreeMap<(String, String), Vec<ColumnInfo>> = BTreeMap::new();
@@ -282,6 +290,7 @@ async fn get_columns_postgres(
                     &unique_map,
                     &varchar_values,
                     &jsonb_keys,
+                    gp_extras.as_ref(),
                     &mut output,
                 );
             }