feat: Systema membrane boundary and capability radius integration docs

[mdheller]

Closes #28

• docs/integration/systema-membrane-boundary.md: activation, side-effect, and storage membranes documented with admitted/blocked/transformed/logged/witnessed/revoked behavior matrix
• docs/integration/systema-capability-radius.md: R0–R5 radius levels mapped to Agent Machine contracts (capability-declaration, policy-admission, activation-decision, lifecycle)
• examples/membranes/agent-machine-activation-membrane.example.json: three cases (admitted, blocked, revoked)
• examples/reachability/agent-machine-capability-radius.example.json: R0, R2, R4 profiles