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

Bắt đầu với các tiêu chí chấp nhận của issue và cuộc thảo luận review #2504, đặc biệt là cách định khung spec-forward và mang tính lịch sử «once-shipped». Sử dụng workflow /constitution để soạn memory/constitution.md, bao quát khi nào SDD được áp dụng và khi nào không, không thêm hạn ngạch hoặc các cam kết downstream. Được xem là hoàn tất khi tài liệu duy nhất tồn tại trong main và đáp ứng từng tiêu chí được liệt kê.

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.