fix: keep sidebar responsive during PR creation

[cookerpapa]

Creating a PR blocks the sidebar while synchronous git and gh commands wait for network operations. The UI cannot process input or update its activity indicator during remote checks, push, or PR creation.

Run the command chain asynchronously with execFile and await it in the action handler. Preserve argument arrays, closed stdin, remote selection, existing-PR recovery, and error reporting.

Fixes #91.

Validation:

• 31 focused tests passed, including real child processes that check event-loop responsiveness during a pending push. The regression fails on unchanged main.
• TypeScript, runtime parity guard, and diff checks passed.
• Full suite: 721 passed, 19 skipped, 2 failed. Both failures reproduce on unchanged main: paneRestore.test.ts (restored command expectation) and integration/gitOperations.test.ts (AI commit message mock).

Tests use local fake git/gh executables and do not contact GitHub.