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/server/src/utils/worktree.ts

@@ -901,7 +901,7 @@ export async function isPaseoOwnedWorktreeCwd(
     };
   }
 
-  const worktreesRoot = join(paseoHome, "worktrees", parts[0]!);
+  const worktreesRoot = join(paseoHome, "worktrees", parts[0]);
   return {
     allowed: true,
     ...(repoRoot !== undefined ? { repoRoot } : {}),