Docs: add enterprise deployment topology

[dinpd]

Closes #67

Summary

• adds a human-readable enterprise Agent Action Control topology under docs/
• introduces an Enterprise Deployment Topology README section at the transition from developer adoption to enterprise deployment
• explains the gateway enforcement point, AgentPass decision service, durable safety state, provider authorization, and evidence model
• preserves the detailed enterprise-managed MCP authorization SVG as a linked technical reference

Acceptance and validation

• verified all 42 repository-local README link and image targets resolve
• confirmed the committed PNG decodes as a 3840×2520 PNG
• visually inspected the committed image for readable labels, complete boundaries, and unclipped content
• confirmed docs/enterprise-managed-auth-runtime-authorization.svg remains linked from the README
• ran git diff --check
• reviewed the complete two-file diff for scope and wording

The visual was also reviewed and approved by the requester before integration.

Security and risk review

Documentation-only change; no runtime behavior, permissions, dependencies, schemas, or data handling changed. Reviewed the README diff and PNG string metadata for local filesystem paths, clipboard identifiers, private-key markers, GitHub tokens, AWS access-key patterns, and other obvious secret material; no matches were found. The repository does not have gitleaks installed locally, so no dedicated secret-scanner result is claimed.