[Feature]: Adopt the CE-migration ADRs and build the integration-test scaffold (Phase1)

User Story

As an interCEde maintainer and contributor,
I want the migration ADRs merged and a shared integration-test scaffold in place,
So that the in-flight backend PRs (#6, #7, #8) harmonize onto one agreed layout instead of
diverging, and every later stack has a template to slot into.

Feature Description

This issue tracks the two foundational PRs and links both:

1. The ADR PR — #4. Adds docs/adr/ with IC-ADR-001 (Computing Element
   interfaces & the DIRAC → interCEde migration), IC-ADR-002 (Integration testing against
   containerized backends) and IC-ADR-003 (backend credential model — typed credentials,
   declared requirements, provider-supplied), plus the template and index.

2. The scaffold PR (new, branched off #4 ). Lands the tests/integration/ skeleton
   that IC-ADR-002 describes — the target the three in-flight backend PRs converge onto. Stacked
   on the ADR branch (so its diff is scaffold-only); retargeted to main once #4 merges.

   Scaffold contents (Phase 0 — native-binary friendly, no dependency on the not-yet-existing
   intercede package):

   • tests/integration/stacks.yml — manifest seeded with the three stacks' entries so each author
     sees where their line goes;
   • tests/integration/README.md — the harmonization guide / stack contract + checklist;
   • tests/integration/stacks/_template/ — compose.yml skeleton (network, credentials volume,
     HEALTHCHECK block) + config/basic/intercede-client.toml template;
   • tests/integration/stacks/_images/ — Dockerfile conventions (ARG + # renovate: annotation);
   • tests/integration/conftest.py — thin --stack/--config options + capability-skip helper;
   • integration-test markers registered in pyproject.toml
     (remote scheduler arc htcondor slurm ssh destructive_fetch);
   • a skeleton .github/workflows/integration.yml that reads stacks.yml, so authors can
     delete their bespoke per-PR workflows against a real target.

Once this lands, harmonizing a stack = move files under stacks/<id>/ + add a manifest entry +
follow the checklist — not "invent structure" or "rewrite against interCEde".

Definition of Done

• PR #4 merged — docs/adr/{IC-ADR-001,IC-ADR-002,IC-ADR-003,IC-ADR-XXX_template,index} on main
• Scaffold PR merged (based on adr_migration, retargeted to main after #4)
• tests/integration/ skeleton in place: stacks.yml (3 seeded stacks), README.md
  checklist, stacks/_template/, stacks/_images/ conventions, conftest.py, markers in
  pyproject.toml, skeleton integration.yml
• A stack is runnable locally with the documented two-liner
  (docker compose … up --wait + pixi run pytest … --stack=<id>)
• Harmonization guide linked from #5, #, # so all three authors
  work from the same target
• Follow-up issues filed for the deferred consolidation: manifest-driven CI matrix (fleshing
  out integration.yml), prebuilt GHCR images, Renovate two-lane + version window, and the
  arc-condor / ssh-condor stacks

Alternatives Considered

• Merge the three prototypes as-is, consolidate later. Rejected: without a shared target they
  re-diverge on the exact axes they already differ on (systemd vs foreground, Podman vs Docker,
  Ubuntu vs EL9), making the later consolidation larger than doing the scaffold first.
• Fold the scaffold into the ADR PR (#4). Rejected: keeps the ADR reviewable as pure docs and
  lets the scaffold evolve without re-opening the ADR; the stacked-PR link keeps them associated.
• Full protocol-based contract suite now. Deferred — the intercede Protocols
  (IC-ADR-001) don't exist yet, so Phase 0 tests use native client tools.

Related Issues

Implemented by: #4 (ADR docs) and the new scaffold PR
Related to: #5 (htcondor stack), # (PR #7), # (PR #8) — all three
harmonize onto this scaffold
Blocks: #5, #, # (they need the scaffold as their target)

Additional Context