feat(macos): make agent window presentation configurable

[SarthakWade]

Summary

• Open CLI-launched macOS agent windows behind the current app by default so automation does not steal focus.
• Add persistent startup-presentation configuration with validated background and foreground values.
• Keep --background and --foreground as one-launch overrides without reordering an existing host.
• Preserve normal foreground behavior for direct GUI launches and reject unsupported presentation controls on Linux.
• Document the contract and architecture decision across the CLI help, phase contract, README, changelog, and web docs.

Validation

• pnpm test
• pnpm test:runtime
• pnpm test:e2e:mac
• pnpm test:e2e:linux
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• pnpm dlx prettier --check CHANGELOG.md README.md apps/headless/docs/P0.md apps/web/components/docs-markdown.ts docs/roadmap/architecture-decisions.md
• zsh -n apps/headless/Tests/macos-e2e.sh
• sh -n apps/headless/Tests/linux-e2e.sh
• git diff --check