Replaces the F-018 sequencing shim at `src/app.rs:1030` with a real
async migration of the two paths the audit flagged for blocking the
Tokio runtime.

Hook path:
- `hook_dir` → `async fn`, uses `tokio::process::Command` for
  `git rev-parse --git-dir` (with the explicit `std::process::Output`
  type annotation per CLAUDE.md gotcha).
- `hook_path`, `hook_install`, `hook_uninstall`, `hook_status`,
  and `handle_hook` are all `async fn` and `.await` the chain.
- `Commands::Hook { action }` dispatch in `handle_command` now
  `.await`s `handle_hook`.

Clipboard path:
- `arboard` is a synchronous library (not a subprocess), so the
  correct fix is `tokio::task::spawn_blocking` rather than
  `tokio::process::Command`.
- `copy_to_clipboard` now takes `String` (owned, since it crosses the
  blocking-task boundary) and returns the result via `await`.
- The `--clipboard` flow in `generate_commit` calls `await` on the
  helper.

The `#[allow(clippy::disallowed_methods)]` shim added in #21 (F-018)
is removed — the lint now has nothing to suppress in this file.

Closes #11.