Add guided Homebrew setup

[deverman]

User outcome

A Homebrew user can run one guided CLI flow to find the bundled OmniFocus bridge, preview every detected destination, install safely after confirmation, see MCP client settings, and check readiness after restarting OmniFocus.

What changed

• add focusrelay setup with dry-run, non-interactive, optional client snippets, and opt-in readiness checking
• centralize plug-in discovery, version checks, copy/restore safety, and already-current detection in Swift
• reduce scripts/install-plugin.sh from 199 lines to an 8-line wrapper around the shared implementation
• cover Homebrew/custom/iCloud/sandbox/legacy path selection, confirmation parsing, dry run, mismatch, copy failure, already-current, symlink paths, and readiness reporting with Swift Testing
• align README, publishing guidance, changelog, and the roadmap
• prepare matching Homebrew caveats in Use guided FocusRelay setup in Homebrew caveats homebrew-focus-relay#5; that draft must wait for a release containing this command

Validation impact

package

Acceptance journey

A Homebrew user runs focusrelay setup, reviews the detected OmniFocus app plus plug-in source and destinations, confirms the update (or explicitly uses non-interactive mode), fully restarts OmniFocus, checks readiness, and copies the printed command plus serve argument or requested client snippet into their MCP client.

Evidence

• swift run focusrelay-dev validate --impact package
• swift run focusrelay-dev check-markdown-links
• git diff --check
• repository installer smoke: both installed copies already current
• live readiness: bridge and binary 0.0.0-dev matched
• release-package dry run: adjacent bundled plug-in detected and both installed copies reported current

Refs #92