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/commands/provider/ls.ts

@@ -76,7 +76,7 @@ export async function runLsCommand(
         provider: entry.provider,
         label: entry.label ?? entry.provider,
         status: entry.status === "ready" ? "available" : entry.status,
-        enabled: entry.enabled === false ? "Disabled" : "Enabled",
+        enabled: !entry.enabled ? "Disabled" : "Enabled",
         defaultMode: entry.defaultModeId ?? "default",
         modes: (entry.modes ?? []).map((mode) => mode.label).join(", "),
       })),