docs(project): complete public installation guidance

[LMLiam]

Goal

Document both installation paths, the privilege boundary, Gatekeeper approval, use, recovery, and contribution workflow.

Acceptance criteria

• Match claims to implemented behaviour.
• Separate Homebrew and DMG requirements.
• Explain macOS 13 CLI and macOS 26 companion support.
• Include setup, Control Centre placement, troubleshooting, and removal.
• State that the DMG is ad-hoc signed and not notarised.
• Add contribution, security, and support policies.
• Pass Markdown lint without internal process notes.

Owned files

README.md, CONTRIBUTING.md, SECURITY.md, SUPPORT.md, Markdown lint configuration, and relevant files under docs/.

Dependencies

Stable command and companion behaviour.

Verification

npx --yes markdownlint-cli2 '**/*.md' '#.build'
git diff --check