Release nvca and the deploy/stacks subprojects from main #1329

Problem

Four subprojects opt out of the shared release automation by declaring version_file and dev_prerelease in tools/ci/github-release-subprojects.json:

• nvca (src/compute-plane-services/nvca)
• nvcf-compute-plane-stack (deploy/stacks/nvcf-compute-plane)
• nvcf-self-managed-stack (deploy/stacks/self-managed)
• nvcf-observability-stack (deploy/stacks/observability)

For these, a push to main only bumps a -dev.N prerelease read from a VERSION file. A stable version appears only when a commit lands on a release branch cut by github-release branch-cut, which also opens a PR advancing the VERSION file to the next train.

Every other subproject takes its version from semantic-release over Conventional Commits on merge to main. Carrying two release models means contributors have to know which one applies to the subproject they touched, and the docs have to describe both.

Proposal

Retire the version-file model so every registered subproject releases the same way:

• Drop version_file and dev_prerelease from the four entries.
• Delete the four VERSION files. Nothing outside the release script reads them.
• Remove the branch-cut command, its two workflow_dispatch jobs, and the release-**/v* push trigger from release-tags.yml.
• Cut releases from the default branch only. A patch on a release-* maintenance branch is tagged by hand.

Version continuity is preserved by anchoring each line at the version its VERSION file last held: nvca 3.3.0, compute-plane 0.2.0, self-managed 0.8.0, observability 0.0.0. initial_version alone is not enough, because the floor is only synthesized for a service with no tags and each of these has hundreds of -dev.N tags.