[EPIC] Documentation baseline

[kirich1409]

Context

Minimum documentation set required for the 0.1 public release: README, CONTRIBUTING, SECURITY.md, docs/architecture.md.

Goals

• README with install / quickstart / safety overview.
• CONTRIBUTING with dev setup, plugin author quickstart.
• SECURITY.md with threat model, accepted risks, disclosure channel.
• docs/architecture.md with Plugin API layout and lifecycle diagrams.

Child tasks

Will be added as sub-issues after task batch.

Exit criteria

• All four files present, cross-linked from README.

Related documents

• Research report §Accepted risks
• Research report §Pre-1.0 checklist