docs: getting_started.md brew install instructions missing tap step and formula has placeholder SHA256

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Problem

getting_started.md documents a Homebrew installation method that would fail for two reasons:

### On macOS (via Homebrew)
brew install vajax

Issue 1: No published tap

vajax is not in homebrew-core. The formula is in the repo at homebrew/Formula/vajax.rb but has not been published to any public Homebrew tap. Users would need to either:

• brew tap ChipFlow/vajax (if a tap were published), or
• brew install --formula homebrew/Formula/vajax.rb from the local checkout

Without a brew tap step first, brew install vajax will fail with "No available formula".

Issue 2: Placeholder SHA256 in formula

homebrew/Formula/vajax.rb contains:

sha256 "PLACEHOLDER_SHA256"

This would cause brew install to fail with a SHA mismatch even if the tap were available.

Recommendation

Either:

1. Remove the Homebrew section until the formula is properly published, or
2. Add the brew tap step and update the SHA256 once the formula is ready

File: docs/getting_started.md, lines 22-27