DOC: document the commit-message prefix convention

[nicolasdmolina]

User problem

Commit and pull-request titles in this repository already use NumPy-style prefixes (FIX:, DOC:, MAINT:, …), but the list was never written down. Synonyms have drifted (FIX:/BUG:, DOC:/DOCS:, TST:/TEST:, MAINT:/IMP:/INFR:), so a contributor has no way to pick correctly.

Scope

Document the canonical prefix list in the contributing guide:

• one-line definition per prefix
• synonyms named as deprecated
• mapping to QEP-2 Type labels where one exists

CONTRIBUTING.md now points at that section. This does not add a pull-request template; that file is not in-tree and is tracked as #892. When #892 lands, the template should reference this convention.

Design choice

RFC: stays the refactor prefix (it is already in use) rather than introducing a new REF: synonym. CI: is listed as canonical because it is the most common prefix on main and maps to the same QEP-2 Type as INFR: (infrastructure), with a narrower one-line definition (workflow files vs broader tooling).

Closes #902

[quantecon-services]

Hi @nicolasdmolina — a quick process note while this is in the review queue.

QuantEcon's Code of AI Use (QEP-5, QuantEcon/qeps#13) doesn't restrict AI-assisted contributions, but it does ask that meaningful AI involvement be disclosed. We'd like it in two places: a trailer in the commit message, and a one-line note in the PR description.

If AI tools were involved here, please add the trailer:

Assisted-by: <tool name>

(git commit --amend, then force-push the branch.) And add a line to the PR description saying which tools were used and for what. If the work was entirely hand-written, a quick note here saying so is equally useful.

The intent is that reviewers know what to expect, and that a human has chosen the task, checked the result, and will own the follow-up discussion.

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