Add the SQA and QC framework and project instructions

Mount the framework as a submodule at framework/ (v0.4.0), add AGENTS.md and
the installed skill copies for Codex CLI (.agents/skills) and Claude Code
(.claude/skills).

.agents/skills/artifact/references/DCD.md

@@ -0,0 +1,27 @@
+# Design Class Diagram (DCD) (DCD)
+
+Cite in `CrossReference` only if the instance exists (`new-artifact.sh` checks this for you):
+
+- **DM** — Concepts each design class refines — names must stay consistent
+- **SD** — Messages that become method signatures
+- **ERD** — Forward: persistence of the classes' attributes
+
+## Required sections (after Metadata / Version History)
+
+1. **Purpose and Scope**.
+2. **Diagram** — PlantUML class diagram: visibility markers (`+` `-` `#`)
+   on every member; association vs aggregation vs composition vs dependency
+   used per true ownership/lifecycle; multiplicity and navigability on every
+   association.
+3. **Class Table** — `Class | Refines (Domain Model concept) |
+   Responsibility (one sentence) | Attributes | Operations`. SOLID applied;
+   no god classes; names consistent with the Domain Model.
+4. **Method Traceability** — `Method signature | Operation Contract / SD
+   message`; every method traces to one.
+5. **Pattern Annotations** — `Pattern | Classes | Rationale`, explicit.
+6. **Dependency Check** — note confirming no circular class/package
+   dependencies (or an explicit justification).
+
+## Terminology
+
+Class and attribute names are the IT terms from the dictionary (`DICT`), not the PO terms the Domain Model uses.