One line installs or updates TuiCode on any Linux distro #415

Context

On Linux the README offers Homebrew or a hand-downloaded tarball that never updates. This task adds a one-line install that works on any distro and updates by running it again. Pitch: #44. Needs #414 merged first, so the binary it installs is known to start on the distros it targets.

Acceptance criteria

• curl -fsSL https://raw.githubusercontent.com/mentaldesk/TuiCode/main/install.sh | sh on Debian 12, Ubuntu 22.04, Fedora and Arch (x64 and arm64) installs the latest release's tuicode to ~/.local/bin, without sudo.
• It prints what it's doing, as in the pitch's mockup: the version and architecture, download, sha256 verified, where it installed, and Re-run this script to update.
• If ~/.local/bin isn't on PATH, it says so and shows the line to add.
• Running it again when a newer release exists replaces the binary; when it's already current it says so and leaves it alone.
• A checksum mismatch, an unsupported architecture or OS (including macOS, pointing at Homebrew), or no network each stop with a one-line explanation and nothing half-installed.
• The README's install section has a Linux block with the one-liner, and no longer sends Linux users to Homebrew first.

Tests

• A CI job that runs install.sh against the latest published release in clean debian:12, ubuntu:22.04, fedora and archlinux containers, then runs tuicode --smoke.
• The checksum failure path (a tampered or missing .sha256) exits non-zero and leaves no binary behind.
• shellcheck clean, POSIX sh (no bash-isms), works with only curl or only wget present.

Out of scope

• .deb/.rpm packages and the apt repo (their own tasks).
• System-wide installs to /usr/local/bin, uninstall commands, or installing a specific older version.
• Running on macOS or Windows.
• Terminal config drops (like the iTerm2 profile).