README: Homebrew instructions point at `jetbrains-junie` tap and miss needed trust step

Following the README's Homebrew instructions on macOS doesn't go smoothly right now, and some of it is the instructions themselves rather than the formula.

The README's **macOS (Homebrew)** block says:

```sh
brew tap jetbrains-junie/junie
brew install junie
```

but the docs site (https://junie.jetbrains.com/docs/junie-cli.html) says:
```sh
brew tap jetbrains/junie
brew update
brew install junie
```

On top of that, `github.com/jetbrains-junie` 404s, and `jetbrains-junie/homebrew-junie` only resolves via a GitHub redirect to `JetBrains/homebrew-junie`. So the README is pointing at an incorrect (legacy?) namespace that doesn't exist on its own.

Two issues when you actually follow the README:

1. Under Homebrew 6's tap-trust, tapping a third-party tap throws `Refusing to load formula ... from untrusted tap` and you have to `brew trust` it first, which isnt mentioned. Combined with the `jetbrains-junie` name and the 404 org, it's genuinely hard to tell at a glance whether you're looking at the real thing or a typosquat. I ended up confirming it by checking the commit author (TeamCity) and following the redirect before I trusted it.
2. The formula it installs is currently broken on Apple Silicon (see my issue JetBrains/homebrew-junie#2), so even after you get past the tap step it won't launch.

Also:

Leaning on that GitHub redirect staying alive is also a small namespace-takeover surface. Since `jetbrains-junie` seems like it isn't used anymore, the name could free up at some point (even if that means someone at JetBrains can't find a use for it and manually releases it), and if anyone re-registers it, `brew tap jetbrains-junie/junie` would resolve to whoever holds it. Pointing everything at the verified `jetbrains` org removes that dependency entirely.

### Suggested fix
Bring the README in line with the docs site:

```sh
brew tap jetbrains/junie
brew update
brew install junie
```

and maybe a one-liner noting that on Homebrew 6+ the first install will ask you to trust the tap (`brew trust jetbrains/junie`).

Happy to send this as a PR against the README if that's easier on your end.