Incus: make prompts visible for end-user - #73

✍️ Description

example docker, no read -rp is displayed, incus exec has no pty, so bash printed no read -p prompt: the question was invisible and its EOF aborted the install under catch_errors. A read shim in
the container prints the prompt itself and treats EOF as the default, and prompt_confirm no longer needs /dev/tty to ask.

2nd.)
The arm64 confirmation and the apt mirror fallback tested for a tty or opened /dev/tty directly, so under Incus the first aborted every unverified arm64 install with exit 106 and the second could never be offered.

🔗 Related Issue

Fixes #

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected (Proxmox VE and/or Incus, as applicable).
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.
• API.txt regenerated – If a function signature or behavior changed, API.txt was updated to match.

🤖 AI Assistance (X in brackets)

If you used an AI tool (GitHub Copilot, Claude, ChatGPT, etc.) to write or generate any code in this PR, you must confirm compliance below.
Select exactly one option.

• No AI used – Code was written without AI assistance.
• AI was used – The output has been reviewed and corrected, and I take responsibility for it matching this repo's conventions.

🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing behavior in a way that may require updates in every downstream script.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, docs/, CONTRIBUTING.md, or other docs.