Automated nightly/dev releases

[ErikBjare]

We are preparing for a new release soon, it would be good to do what we did for the gptme repo and set up CI that automates nightly/dev releases every now and then (every other week? monthly?). That would let users help us test the new release before we make a stable one.

@TimeToBuildBob Please review the workflows and PRs that set it up for gptme, and take inspiration from there to do the same here.

[TimeToBuildBob]

Implemented this in #1217.

What I changed:

• added a scheduled/manual dev-release.yml workflow that cuts the next beta prerelease tag when master has new green commits
• reused the existing tag-triggered build workflows instead of duplicating packaging logic
• made tag builds always publish as draft prereleases, and preserved the exact tag in macOS artifact filenames

The schedule is biweekly (Thursday 12:00 UTC on even ISO weeks), with manual dispatch support for patch-line or minor-line prereleases.

[ErikBjare]

#1217 was merged, this issue was closed, but reopening until we've verified the first automated/scheduled release.

[ErikBjare]

Triggered it manually via workflow_dispatch: https://github.com/ActivityWatch/activitywatch/actions/runs/23379388417