[Feature]: Dogfood /constitution on spec-kit (add memory/constitution.md)

### Problem Statement

Spec-kit ships `/constitution` but the repo itself has no `memory/constitution.md` in `main`. There is no upstream artifact showing the workflow runs on the project that ships it — the dogfooding gap raised in discussion #2504. A maintainer (@mnriem) gave a green light to try it there, with the framing that specs in this repo should be **spec-forward, historical-once-shipped** (no living-doc maintenance burden against a fast-moving codebase).

### Proposed Solution

Add `memory/constitution.md`, drafted via the `/constitution` workflow. It codifies:

- **When SDD applies here**: non-trivial scope, ambiguous design, cross-cutting impact, security/correctness stakes.
- **When it does not**: plumbing, refactors, catalog updates, doc fixes.
- **Stance**: spec-forward; historical once shipped (frozen snapshot, not maintained).

Single document. No quota, no cadence, no downstream commitment to spec any particular future PR.

### Alternatives Considered

- Retroactive specs on shipped PRs (e.g. #2393): parked — does not fit "spec-forward."
- Per-feature triage on every PR: re-litigates the question on every change, more friction.
- Status quo: dogfooding gap remains.

### Component

Documentation

### AI Agent (if applicable)

Not applicable (repository governance, not agent-specific)

### Use Cases

1. Contributor unsure whether their PR warrants a spec consults the constitution instead of asking per-PR.
2. Reviewer points to it to flag a fitting candidate ("cross-cutting and security-sensitive — want to spec this?").

### Acceptance Criteria

- [ ] `memory/constitution.md` exists in `main`
- [ ] Drafted via the `/constitution` workflow
- [ ] Codifies criteria for when SDD applies to spec-kit work
- [ ] Codifies the "spec-forward, historical-once-shipped" stance per #2504
- [ ] No downstream commitments added (no quota, no required follow-up specs)

### Additional Context

Discussion #2504, in particular @mnriem's comment giving the green light and the spec-forward framing. PR follows.

Commencez par les critères d’acceptation de l’issue et la discussion de revue #2504, en particulier le cadrage spec-forward et historique « once-shipped ». Utilisez le workflow /constitution pour rédiger memory/constitution.md, en couvrant les cas où SDD s’applique et ceux où il ne s’applique pas, sans ajouter de quotas ni d’engagements en aval. C’est terminé lorsque l’unique document existe dans main et satisfait chacun des critères listés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.