DOC: Document the commit-message prefix convention

Problem

The project follows a numpy-style commit prefix convention that is written down nowhere, and it has drifted into synonyms as a result. Across the last 120 commits: FIX: 12, RLS: 11, MAINT: 9, RFC: 7, DOC: 7, ENH: 3, PERF: 2, BUG: 2, TST:/TEST: 1 each, INFR:/IMP:/DOCS: 1 each.

FIX: and BUG: are the same thing; so are DOC:/DOCS: and TST:/TEST:; MAINT:, IMP: and INFR: overlap. A contributor has no way to pick correctly.

Worth aligning the vocabulary with the QEP-2 Type labels where they correspond — MAINT: ↔ maintenance, INFR: ↔ infrastructure, RFC: (or a new REF:) ↔ refactor — so a commit prefix and its issue's Type label tell the same story. That alignment is the main reason to bother.

Acceptance criteria

• The canonical prefix list documented in CONTRIBUTING.md or the contributing guide
• Each prefix has a one-line definition; synonyms named as deprecated
• The mapping to QEP-2 Type labels stated where one exists
• The PR template (community-files issue in this milestone) references the convention

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Start with CONTRIBUTING.md or the contributing guide, then inspect the PR template and the QEP-2 Type labels referenced in the issue. Use the listed commit-prefix history to define the canonical vocabulary and deprecated synonyms; done means the convention, definitions, Type mappings, and PR-template reference are documented.