Hook `async` property silently ignored — postToolUse hooks block tool completion

## Describe the bug

The `async` property on command hooks in `hooks.json` is silently ignored. Hooks with `"async": true` still block tool completion because `executeHooks()` unconditionally `await`s every hook in a sequential `for` loop.

Claude Code [documents `async`](https://docs.anthropic.com/en/docs/claude-code/hooks) as a supported hook field:

> **`async`** — If `true`, runs in the background without blocking.

Since plugins share the same `hooks.json` format across both Claude Code and Copilot CLI, authors reasonably expect `"async": true` to work. Instead, it is parsed by the Zod schema (via `.passthrough()`) but never read by the execution path.

## Impact

In a real session, a `postToolUse` hook with `"async": true` blocked for **13.5 seconds** on first invocation because it triggered a package auto-update (resolving 419 packages via `uv`). This added ~14s of latency to the first tool call of the session despite the hook being marked async.

**Timeline from session events:**
```
18:57:09.756 hook.start postToolUse
18:57:22.186 [hook stdout] Resolved 419 packages in 6.91s
18:57:22.803 [hook stdout] Installed 2 packages
18:57:23.216 hook.end ← 13.46s blocked
```

## Root cause

In `sdk/index.js`, the `executeHooks` function (`Yg` in minified source):

```javascript
async function executeHooks(hooks, input, logger, hookType, emitter) {
// ...
for (let hook of hooks) {
let result = await hook(input); // ← always awaits, ignores async flag
// ...
}
}
```

And in `processToolExecutionResult`:
```javascript
await executeHooks(this.getEffectiveHooks()?.postToolUse, ...) // blocks tool completion
```

The `async` property from the hook config is never propagated to the execution layer.

## Expected behavior

When a hook specifies `"async": true`, it should run in the background without blocking tool completion — matching Claude Code's documented behavior. Ideally also support `asyncRewake` for parity.

## Workaround

Plugin authors can append `&` to the shell command to background it manually, but this loses stdout/stderr capture and exit code handling.

## Environment

- Copilot CLI: 1.0.40-2
- OS: macOS (Darwin arm64)

## Related issues

- #2893 — hooks serialized + timeout fail-open (related hook execution model concerns)