chore: move Cassandra build pipeline to dedicated cassandra-builds repo

[rustyrazorblade]

Summary

• Every per-build Cassandra release (cassandra-<version>-<sha>) was creating a permanent git tag on this repo, one per build per tracked branch, forever. That whole pipeline now lives in https://github.com/rustyrazorblade/cassandra-builds instead — a repo whose entire purpose is hosting Cassandra build artifacts.
• Removed .github/workflows/build-cassandra-ref.yml, .github/workflows/build-cassandra-set.yml, .github/cassandra-image/, the testCassandraBuildPlan/testCassandraResolveRef Gradle tasks, and the corresponding packer-test.yml job + path filters.
• packer/cassandra/cassandra_versions.yaml now points 5.0-HEAD, 6.0-HEAD, and trunk at cassandra-builds' nightly release, and adds a new 6.0-rustyrazorblade-HEAD entry for the fork branch that was tracked there.
• Updated .github/CLAUDE.md, docs/development/building-cassandra-refs.md, docs/reference/commands.md, docs/user-guide/installing-cassandra.md, and docs/user-guide/tutorial.md accordingly.
• cassandra-builds is seeded and already running: nightly matrix builds apache/cassandra (5.0, 6.0, trunk) + rustyrazorblade/cassandra (cassandra-6.0-rustyrazorblade). See that repo's README for how to add more branches or trigger a one-off build.
• Existing cassandra-* tags already on this repo are untouched — only future per-build tag creation stops.

Supersedes #871 (which added the rustyrazorblade fork branch to this repo's now-removed nightly matrix) — recommend closing it.

[claude]

Reviewed. This is a clean, well-scoped removal — I checked out the merged state and confirmed no dangling references remain (no leftover mentions of cassandra-image, build-cassandra-ref, build-cassandra-set, testCassandraBuildPlan, or testCassandraResolveRef anywhere in packer/, docs/, or the Gradle build). The packer-test.yml and build.gradle.kts changes correctly remove the now-orphaned test job/tasks together with the scripts they tested. Docs (.github/CLAUDE.md, docs/development/building-cassandra-refs.md, docs/reference/commands.md, docs/user-guide/installing-cassandra.md, docs/user-guide/tutorial.md) are all consistently updated, including the new 6.0-rustyrazorblade-HEAD version entry.

One thing worth a look, not blocking:

Stale OpenSpec change directory — openspec/changes/cassandra-branch-publish/ (proposal, design, tasks) documents the exact workflow this PR deletes, and it was never archived into openspec/specs/ (there's no corresponding spec there). Per this repo's own convention, OpenSpec changes should be archived once implemented; since the feature they describe has now been ripped out entirely, this change directory is left describing infrastructure that no longer exists. Worth either archiving it as superseded/abandoned or deleting it in a follow-up so openspec/changes/ doesn't mislead someone running openspec-apply-change later.

Minor/informational, not a blocker:

• cassandra_versions.yaml now points at rustyrazorblade/cassandra-builds's nightly release tag for 5.0-HEAD, 6.0-HEAD, 6.0-rustyrazorblade-HEAD, and trunk. This is an external, unauthenticated dependency (install_cassandra.sh does a plain curl, no token) — reasonable since the repo is public per the description, but worth keeping in mind that lab installs of these four versions now hard-depend on that repo's release cadence/availability rather than this one's.

No security, correctness, or test-coverage concerns — this is a straight deletion of a self-contained subsystem plus a config/doc update, and the diff shows nothing left half-migrated.

[github-actions]

Code Coverage

Total Project Coverage	73.48%	❌