docs(project): complete public installation guidance

[LMLiam]

Summary

• Add separate Homebrew and DMG installation instructions.
• Document commands, timers, Control Centre, recovery, and removal.
• Expand the existing contribution, support, security, and privilege policies.
• Add repository-wide Markdown lint configuration.

Issues

Closes #7.

This PR is stacked on #18 because the documentation describes the packaged companion.

Behaviour and security

This PR does not change runtime behaviour. The documentation defines the exact privileged paths, file modes, helper commands, battery-only pmset boundary, ad-hoc signing, checksum check, and private vulnerability process.

The installation guide states that the first public release and formula are not available. Later release issues will remove this notice only after publication works.

Verification

• scripts/check.sh
• npx --yes markdownlint-cli2 '**/*.md' '#.build'
• npx --yes markdown-link-check --quiet <document> for each new or changed public guide
• git diff --check
• Manual CI run 30494421333

Screenshots

Not applicable.

Checklist

• scripts/check.sh passes.
• User and contributor documentation matches the change.
• Documentation uses ASD-STE100 Simplified Technical English.
• Every commit uses verb(area): description.
• Privileged behaviour is covered by safe tests rather than live system mutation.

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