[Feature]: Manifest-driven integration CI matrix + prebuilt stack images on GHCR

## User Story

As an interCEde CI maintainer,
I want one manifest-driven integration workflow and prebuilt backend images,
So that adding a stack is a `stacks.yml` entry (no workflow edits) and PR CI pulls images instead
of spending 5–10 min rebuilding RPM-based images per job.

## Feature Description

Consolidation of the three harmonized prototype stacks (IC-ADR-002 §3, §5):

1. **Flesh out `integration.yml`** from the scaffold skeleton: a `matrix` job reads `stacks.yml`
and emits the `{stack × config × version}` JSON; per-stack jobs run
`docker compose … up --wait` → `pytest -m "" --stack= --config=`
(or `compose exec` for `exec_in_container` stacks); `fail-fast: false`; on failure upload
`docker compose logs` + backend log dirs as artifacts. Reusable via `workflow_call` from
`ci.yml`, required for merge.
2. **`build-images` workflow → GHCR**: build and push
`ghcr.io/diracgrid/intercede-testenv/{arc,slurm,htcondor-ce}:` when Dockerfiles or
pinned `ARG`s change, plus a weekly rebuild for base-image updates. PR CI pulls by digest.
Daemons foreground under a minimal supervisor — never systemd (§3).

## Definition of Done

- [ ] `integration.yml` generates its matrix from `stacks.yml`; the three bespoke prototype
workflows are deleted
- [ ] Each stack runs as its own job; failure uploads compose + daemon logs as artifacts
- [ ] `build-images` workflow pushes pinned-`ARG` images to GHCR; PR CI pulls, never builds
- [ ] Integration workflow is `workflow_call`-reusable and required for merge
- [ ] Adding a stack = manifest entry + files (verified by adding no workflow lines for one stack)

## Alternatives Considered

- Per-stack bespoke workflows (status quo of the prototypes) — O(stacks) workflow maintenance,
drift between them; rejected by IC-ADR-002 §5.
- Rebuilding images in every PR job — 5–10 min × jobs wasted; rejected by §3.
- One mega-compose job — shared fate, poor attribution; rejected (ADR Rejected Ideas).

## Related Issues

## Additional Context