fix: delete existing manifest before push (Gitea #31964 workaround)

Gitea 1.27 fails with HTTP 500 when pushing a tag that already exists.
Add delete_remote_manifest() to delete the existing manifest via the
Docker registry v2 API before each push.

Implements: REQ-1, REQ-2, REQ-3, REQ-4, REQ-5

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/specs/DEVX-158.md

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+# 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