[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.

issue の受け入れ基準とレビュー・ディスカッション #2504 から始め、特に spec-forward で歴史的な「once-shipped」という位置づけを確認してください。/constitution ワークフローを使用して memory/constitution.md を作成し、SDD が適用される場合と適用されない場合を扱ってください。ただし、クォータや下流のコミットメントは追加しないでください。完了とは、単一のドキュメントが main に存在し、列挙された各基準を満たしていることを意味します。

索引モデルが issue の本文から書いたものです。