feat: agentlayer init as unified onboarding entrypoint (ARB-28)

[iMark21]

Summary

• add agentlayer init as the recommended single-entrypoint command
• auto-route to install on fresh repos and to standardize when AI files already exist
• keep install, standardize, and audit available for advanced/scripted usage
• recut README around the first-use path and make AI-assisted prerequisites explicit
• add CI coverage for init install routing, standardize routing, and cwd default path

Scope notes

• init does not change the internal logic of install, standardize, audit, or generated .ai/ templates
• no refresh command in this PR
• no OpenSpec positioning work in this PR

Acceptance criteria

• interactive init on a fresh repo shows preflight summary and routes to install
• interactive init on a repo with AI files shows detected files and routes to standardize
• non-interactive init with explicit --runtimes routes correctly on fresh and legacy repos
• non-interactive init without --runtimes exits non-zero
• agentlayer --help lists init first as the recommended command
• README surfaces the first-use path before structural detail
• README makes AI-assisted prerequisites explicit
• no behavioral changes to install/standardize beyond being invoked by init

Test plan

• local smoke tests for all ARB-28 acceptance criteria
• GitHub Actions CI includes explicit init routing cases
• verified --dry-run, help output, and cwd default path behavior