fix(ci): scope nightly release notes to commits since last release

[NonPolynomialTim]

Problem

The rolling nightly prerelease notes grow without bound — every nightly re-lists the entire commit history rather than just what's new.

Root cause: publish used generate_release_notes: true on the non-semver nightly tag. GitHub's automatic base-selection can't order nightly against the repo's tag soup (v0.1…v2.0.0 plus legacy OpenXcomCoop_*), so it diffs from ~repo start. Deleting/recreating the nightly release each run didn't help — GitHub regenerated the same unbounded list.

Fix

Pin the release-notes diff base to the last official release instead of relying on GitHub's auto base-picker:

• meta job exports a new lastTag output = git describe --tags --abbrev=0 --match "v[0-9]*" → v2.0.0 today. Matches v<digit> to skip legacy junk tags; empty on a tagless repo.
• Publish now sets generate_release_notes: false. Releases still use the curated CHANGELOG.md body; nightly is published body-less.
• The notes step now generates the list itself via gh api …/releases/generate-notes -f previous_tag_name=$lastTag, then runs the existing "New Contributors" + by @author strip. Falls back to GitHub's default base when lastTag is empty.

Net: nightly notes span <last v* release>..HEAD and reset at the next official release tag — i.e. "everything since the last release."

Notes

• Nightly-path only; the versioned-release path is unchanged (still curated CHANGELOG body).
• Can't be dry-run via workflow_dispatch (non-tag ref → nightly channel → would move the live nightly tag). Exercised on the next push to main. The existing if not "## What's Changed" guard still aborts on malformed notes.

🤖 Generated with Claude Code