Refocus innie around the local macOS installer

[Copilot]

innie now treats install.sh as the primary product: a single high-quality local installer for macOS, with CI and release workflows serving as quality gates rather than the main distribution path.

• Distribution model

  • Reframe the repo copy around the local installer as the canonical entrypoint.
  • Clarify that releases package the same installer; CI validates it.

• Docs and contributor guidance

  • Update README language and section naming to center the installer.
  • Align CONTRIBUTING and the PR template with the new “installer-first” model.
  • Tighten the distinction between installer changes and workflow gates.

• Workflow framing

  • Rename CI comments and checklist language to reflect quality-gate usage.
  • Keep the macOS E2E workflow positioned as validation of the installer, not a separate product.