Plan P3 agent scaffold implementation

[lantisprime]

Summary

• add P3 agent/subagent scaffold plan docs
• define agent spec, security model, registration guide, and implementation slice strategy
• document plan reviews, adversarial reviews, and blocker resolutions for trusted registration, project agents, ephemeral agents, and evals

Notes

• planning/docs only; no runtime code
• implementation should follow agents/P3_IMPLEMENTATION_SLICES.md
• canonical workplan memory revised to 20260614-112030-canonical-workplan-updated-with-p3-imple-272e

Validation

• git diff --check