# DEVX-158: Fix build-images workflow: delete existing manifest before push

## Problem
Gitea 1.27 has a known bug (#31964) where pushing a Docker image tag that
already exists in the container registry fails with HTTP 500 "package
version already exists." The build-images workflow has been failing for weeks because
every push to `ci-base:latest`, `ci-quality:latest`, and `ci-full:latest`
hits this error.

## Approach
Add a `delete_remote_manifest` function that deletes the existing manifest
via the Docker registry v2 API before pushing. This works around the Gitea
bug by ensuring the tag doesn't exist when the push starts.

REQ-1: Add `delete_remote_manifest` function using Docker registry v2 API
REQ-2: Call `delete_remote_manifest` before each `docker push` in `push_image`
REQ-3: Pass registry credentials from `main` to `push_image`
REQ-4: Handle errors gracefully — never block the push if delete fails
REQ-5: 100% test coverage for new code

## Test Plan
- Unit tests for `delete_remote_manifest` (success, 404, 500, network error)
- Unit tests for `push_image` with and without credentials
- Verify existing tests still pass

## Deploy Plan
- Merge to master, auto-release new devx version
- The build-images workflow will use the new code on the next run

## Rollback Plan
- Revert the merge commit

## Acceptance Criteria
- [x] REQ-1: `delete_remote_manifest` function added
- [x] REQ-2: Called before each push in `push_image`
- [x] REQ-3: Credentials passed from `main` to `push_image`
- [x] REQ-4: Errors don't block the push (returns True on failure)
- [x] REQ-5: 100% test coverage