Remove unnecessary type assertions across codebase

Add oxlint-tsgolint and configure typescript/no-unnecessary-type-assertion
to flag redundant `!` and `as Foo` casts. Type-aware mode is left off by
default to keep `npm run lint` fast; the rule sits configured for when we
turn type-aware on intentionally. Auto-fix removed ~283 redundant casts;
two manual touch-ups: a real tsgolint false positive in split-container.tsx
and a stale ChildProcess import after a double-cast collapsed.

packages/cli/src/output/table.ts

@@ -137,7 +137,7 @@ export function renderTable<T>(result: AnyCommandResult<T>, options: OutputOptio
     return "";
   }
 
-  const columns = schema.columns as ColumnDef<T>[];
+  const columns = schema.columns;
   const includeHeaders = !options.noHeaders;
   const widths = calculateWidths(data, columns, includeHeaders);