Add 'upgrade' command and automatic updates.

[mm-zacharydavison]

Resolves #87

Adds support for release channels (latest, next) per #198

git-ai upgrade              // checks for updates and upgrades if there is a new version
git-ai upgrade --force      // force re-installs the latest version

Hooks on fetch and push run automated version checks in background processes. Unless auto updates are disabled, it will automatically update as well.

If an update is available and auto updates are disabled, it will print:

A new version of git-ai is available: (1.1.2) → (1.1.3)
Run 'git-ai upgrade' to upgrade to the latest version.

git-ai upgrade delegates to the existing install.sh or install.ps1 script.

• Install testing
• Config testing
• Automated check/install testing
• Update install scripts to stop overwriting config if it already exists (both windows and bash)
• Windows testing or temporarily disable on Windows

[mm-zacharydavison]

Sorry guys, I've got a full plate this week, so don't have time to look at the proposed changes until next week, documenting here in-case anyone else wants to pick it up in the meantime:

1. Add options to disable automated version checks and auto updates (forward-looking feature) via the git-ai config (for enterprises that are using custom releases/MDM)
2. Can we put the version check an async thread that runs on fetch+pull+push pre command books? Similar to this: https://github.com/acunniffe/git-ai/blob/feat/sentry/src/commands/hooks/fetch_hooks.rs#L16. The thinking here is that these commands naturally take a bit longer, so it’s more likely to finish and we won’t add any startup latency. IDEs run fetch in the background a lot, so it should get picked up pretty regularly.
3. When auto updates are disabled, only output the update alert when we’re being run in an interactive terminal (this will make sure it doesn’t impact output parsing for IDEs, etc. that run git/git-ai directly and are reliant on certain output formats). When auto updates are enabled, I don’t think we need to output anything (just update silently).
4. We’ve gotten reports of users getting rate limited on api.github.com endpoints when running on corporate networks. Is there any way to use a plain github.com raw endpoint or something like that?