Refactor CLI UI to OpenTUI and add end-to-end tests

[adrianmg]

Motivation

• Replace the previous interactive layer with an OpenTUI-based UI to provide a richer TUI and move toward a Bun/OpenTUI runtime for prompts and rendering.
• Make CLI flows testable and deterministic by supporting fixture-backed GitHub responses and configurable config paths for CI/e2e runs.

Description

• Replace the prompt layer with an OpenTUI + React-based prompt implementation in src/ui.js, implementing multi/single-select and auth views and a test-mode selection flow using GHPEW_TEST_MODE and env-driven selections.
• Switch the runtime to Bun by updating the shebang in index.js and package.json start script, and convert index.js to dynamic import()s so OpenTUI components initialize at runtime.
• Add fixture support in src/github.js via GHPEW_FIXTURE_FILE so API calls (list/delete/archive) short-circuit during tests, and make config directory path overrideable with GHPEW_CONFIG_DIR in src/config.js.
• Remove interactive deps used previously for prompts/spinners and add @opentui/core, @opentui/react, and react to package.json; add an end-to-end test test/e2e.test.js with fixture test/fixtures/github.json and small UX fixes (src/utils.js, command handlers).

Testing

• Ran the automated test suite with npm test (Mocha), which executed unit and the new e2e flows; result: 9 passing.

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Codex Task