fix(workflow): fold v5.1/v5.2 hotfixes + OC-8 exception into v5.0.20260418 - Structured Phascolarctos

[nullhack]

Summary

• Folds all post-v5.0 changes (v5.1 and v5.2 hotfixes) into a single consolidated v5.0.20260418 release
• Adds OC-8 dataclass/Pydantic/value-object exception across all checklist locations (AGENTS.md, implementation/SKILL.md, refactor/SKILL.md, docs/workflow.md, design-patterns/SKILL.md)
• Adds SELF-DECLARE phase to Step 3 TDD loop diagram and Cycle State phase list in docs/workflow.md
• Appends Core Heuristic — Procedural vs OOP section to design-patterns/SKILL.md

Changes

• AGENTS.md — OC-8 instance variable exception
• .opencode/skills/design-patterns/SKILL.md — Core Heuristic section added
• .opencode/skills/implementation/SKILL.md — OC-8 exception in architecture smell check + self-declaration
• .opencode/skills/refactor/SKILL.md — OC-8 exception in smell table
• docs/workflow.md — OC-8 exception in quality gates + Step 2 smell check + self-declaration templates; SELF-DECLARE phase in Step 3 diagram and Cycle State
• tests/unit/app_test.py — remove stale @pytest.mark.unit marker

Release

After merge: retag v5.0.20260418 at HEAD, delete v5.1.20260418 and v5.2.20260418.