refactor: nightly build workflow and add nightly release workflow

[UnschooledGamer]

[I'll update this soon, it includes maintainability, Testing and checklists - I don't have the list on this device]

[greptile-apps]

Greptile Summary

This PR splits nightly building and releasing into separate workflow paths. The main changes are:

• A reusable nightly build workflow with exported build metadata and artifact names.
• A new nightly release workflow that downloads build artifacts and publishes the nightly prerelease.
• Updated preview-release workflow permissions and reusable build inputs.
• CODEOWNERS coverage for the new nightly release workflow.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
.github/workflows/nightly-build.yml	Refactors the nightly build into a reusable workflow and exports the artifact names used by downstream jobs.
.github/workflows/nightly-release.yml	Adds the nightly release workflow and downloads artifacts using the build workflow outputs.
.github/workflows/on-demand-preview-releases-PR.yml	Narrows workflow permissions and keeps write access on the jobs that update PR state.
.github/CODEOWNERS	Adds an owner entry for the new nightly release workflow.

_{Reviews (2): Last reviewed commit: "Update .github/workflows/nightly-release..." | Re-trigger Greptile}

[UnschooledGamer]

@greptile review again