Update Homebrew install docs for Homebrew 6 tap trust

[quinnmil]

Closes #20

Summary

Homebrew 6.0.0 introduced Tap Trust: third-party taps like scoutapp/tap must be explicitly trusted before Homebrew will load their formulae. This updates the README Homebrew section and adds a CHANGELOG entry.

Changes

• Keep brew install scoutapp/tap/scout-cli as the primary install command, with a note that the fully qualified name trusts only the scout-cli formula.
• Add an "Upgrading from Homebrew 5" note: if brew upgrade shows the "taps are not trusted" warning, run brew trust --formula scoutapp/tap/scout-cli (or brew trust scoutapp/tap for the whole tap), then brew upgrade scout-cli.
• Add a Brewfile / brew bundle install --file=- heredoc form using brew "scoutapp/tap/scout-cli", trusted: true, with a one-line mention of tap "scoutapp/tap", trusted: true for whole-tap trust.
• CHANGELOG: new ## Pending → ### Changed entry.

What was verified

• The Homebrew commit "Trust qualified tap items before install" ships in tag 6.0.0, and the Tap Trust doc states: "Installing a fully qualified formula or cask name trusts only that item." On Homebrew 6.0.22, brew upgrade --dry-run scoutapp/tap/scout-cli prints ==> Trusted formula scoutapp/tap/scout-cli, so the existing one-liner works on Homebrew 6.
• The "taps are not trusted: scoutapp/tap" warning hits users who tapped scoutapp/tap under Homebrew 5 and then run brew upgrade, or who brew tap and then install by short name.
• brew bundle honours trusted: true on tap, brew, and cask entries; the README snippet parses cleanly via brew bundle list --file=-.
• The only in-repo reference to the install command was README.md. The scoutapp/homebrew-tap repo has no README, so nothing to change there. The 0.1.0 CHANGELOG mention of scoutapp/tap is in a versioned section and was left untouched.

Deviation from the issue's proposal

The issue suggested making a whole-tap trusted: true Brewfile the primary instruction. Since fully qualified installs have auto-trusted the formula since 6.0.0, the one-liner stays primary, and the Brewfile example trusts the formula rather than the whole tap, per Homebrew's guidance to "prefer trusting the specific formula, cask or command you need." Whole-tap trust is still documented as the alternative.

Scope from #20

• Update the Homebrew section of README.md
• Update any other docs/onboarding scripts referencing brew install scoutapp/tap/scout-cli (none found besides README)
• Note in CHANGELOG.md

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