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.

package.json

@@ -101,6 +101,7 @@
     "lefthook": "^2.1.6",
     "oxfmt": "0.46.0",
     "oxlint": "1.61.0",
+    "oxlint-tsgolint": "^0.22.1",
     "patch-package": "^8.0.1",
     "playwright": "^1.56.1",
     "typescript": "^5.9.3",