Persona Plugin Architecture — minimal interface + registry

[DevVig]

Summary
Add a minimal, optional plugin interface to register custom personas that provide domain-specific prompts and output validation. Keep clean separation from core and ensure clean uninstall.

Why

• Extensibility for specialized domains without polluting core
• Optional by design; project choice

Scope

• Define base class and registry; load plugins if present

Tasks

• Define a2a/persona_plugins.py with PersonaPlugin base and PersonaRegistry
• Add loader that checks project folder .a2dev/personas/ for plugins
• Methods: get_expertise_prompt(), validate_output(output) -> bool
• Wire registry into role construction points (UX, Architect, QA, etc.)
• Document lifecycle and clean uninstall
• Provide example plugin under examples/personas/

Acceptance Criteria

• Core works unchanged without plugins
• If a plugin folder is present, persona prompt is augmented and validation is applied
• Removing plugin leaves no residual behavior

Risks & Mitigations

• Interface creep -> keep minimal surface, document versioning

References

• suggestion.md (Plugin Architecture for Custom Personas)
• a2a/personas.py, a2a/roles/*