Summary

Fix the nightly install instructions so they tell users to run brew install vigilante-nightly, not brew install --cask vigilante-nightly.

Problem

• The nightly channel currently shows brew install --cask vigilante-nightly in user-facing instructions.
• That guidance is wrong for the nightly package and sends users down the wrong Homebrew install path.
• Stable release instructions should not be changed as part of this issue unless they are incorrectly reused in nightly-specific output.

Context

• The repository currently documents the nightly install path in the README as:
  • brew tap aliengiraffe/spaceship
  • brew install --cask vigilante-nightly
• Stable release documentation still uses brew install --cask vigilante.
• The issue is specifically about nightly builds and any generated or published instructions associated with that channel.
• The implementation should check the source that renders nightly install guidance, not just one README snippet, so the incorrect --cask wording does not keep reappearing in release notes, generated docs, workflow output, or similar surfaces.

Desired Outcome

• Every nightly-specific install instruction tells users to run brew install vigilante-nightly.
• No nightly-specific user-facing instruction tells users to run brew install --cask vigilante-nightly.
• Stable install instructions remain unchanged unless the implementation discovers a shared code path that must be split to preserve the correct stable wording.
• Non-goal: changing the stable tagged-release installation command unless that is required to avoid breaking nightly-specific messaging.

Implementation Notes

• Identify the source of truth for nightly install instructions, including any generated README content, release workflow output, release notes, or templates used to publish nightly guidance.
• Replace nightly-only brew install --cask vigilante-nightly text with brew install vigilante-nightly.
• Preserve the stable-channel wording if it is intentionally different.
• If the nightly packaging scripts or metadata also encode the wrong install mode and that contributes to the displayed instructions, update that source rather than applying a docs-only patch.
• Keep the exact tap name unchanged unless the investigation shows the tap instruction is also wrong.

Acceptance Criteria

• Nightly-facing install instructions use brew install vigilante-nightly.
• Nightly-facing install instructions no longer use brew install --cask vigilante-nightly.
• Stable install instructions are unchanged unless a shared implementation requires a targeted split.
• Any generated output, templates, or publishing workflow responsible for nightly instructions is updated so the wrong command does not regress on the next nightly publish.

Testing Expectations

• Update or add tests for any script, template, or generator that produces nightly install instructions.
• Add regression coverage for the exact nightly install command string so future nightly publishes cannot reintroduce --cask.
• If the only affected surface is documentation, verify with a repository search that nightly references no longer include brew install --cask vigilante-nightly.

Operational / UX Considerations

• Keep the fix narrow to nightly guidance so current stable users are not given conflicting instructions.
• If nightly instructions appear in release artifacts outside the repository, ensure the source in this repository is the one that actually drives those published messages.