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/app/e2e/helpers/terminal-probes.ts

@@ -204,8 +204,7 @@ export async function installTerminalRenderProbe(page: Page): Promise<void> {
         if (next?.write && !next.__paseoRenderProbeWriteWrapped) {
           const originalWrite = next.write.bind(next);
           next.write = (data: string | Uint8Array, callback?: () => void) => {
-            const text =
-              typeof data === "string" ? data : new TextDecoder().decode(data as Uint8Array);
+            const text = typeof data === "string" ? data : new TextDecoder().decode(data);
             probe.writeCount += 1;
             const preview = text
               .replaceAll("\u001b", "\\x1b")
@@ -424,7 +423,7 @@ export async function installTerminalKeystrokeStressProbe(page: Page): Promise<v
       const count = Math.min(from.length, to.length);
       const values: number[] = [];
       for (let index = 0; index < count; index += 1) {
-        values.push(to[index]!.at - from[index]!.at);
+        values.push(to[index].at - from[index].at);
       }
       return values;
     }
@@ -459,7 +458,7 @@ export async function installTerminalKeystrokeStressProbe(page: Page): Promise<v
         const keydownToInputFrame = this.keydowns
           .map((keydown) => firstAtOrAfter(this.inputFrames, keydown.at)?.at ?? null)
           .filter((at): at is number => at !== null)
-          .map((at, index) => at - this.keydowns[index]!.at);
+          .map((at, index) => at - this.keydowns[index].at);
         const inputFrameToOutputFrame = this.inputFrames
           .map((input) => {
             const output = firstAtOrAfter(this.outputFrames, input.at);
@@ -663,8 +662,7 @@ export async function installTerminalKeystrokeStressProbe(page: Page): Promise<v
         if (next?.write && !next.__paseoKeystrokeProbeWriteWrapped) {
           const originalWrite = next.write.bind(next);
           next.write = (data: string | Uint8Array, callback?: () => void) => {
-            const text =
-              typeof data === "string" ? data : new TextDecoder().decode(data as Uint8Array);
+            const text = typeof data === "string" ? data : new TextDecoder().decode(data);
             const event: XtermWriteEvent = {
               at: performance.now(),
               committedAt: null,

packages/app/e2e/terminal-keystroke-stress.spec.ts

@@ -278,7 +278,7 @@ async function measureDaemonBurstEcho(
       timeoutMs: STRESS_TIMEOUT_MS,
     });
 
-    const latencies = sendTimes.map((sentAt, index) => outputTimesByByte[index]! - sentAt);
+    const latencies = sendTimes.map((sentAt, index) => outputTimesByByte[index] - sentAt);
     return {
       inputTextLength: inputText.length,
       inputFrameCount: sendTimes.length,