docs: align engine documentation with current features

[ovasylenko]

What changed

• expand the root feature overview to cover the implemented execution, worker, governance, AI, security, distribution, continuity, and operations capabilities
• make docs/README.md a complete index of the repository's feature guides
• align architecture documentation with node roles, gRPC sessions, governed push delivery, package distribution, and current CLI responsibilities
• update the curated API guide for secure tenant defaults, principals, entitlements, effects, change feeds, remediation, and continuity controls
• make the Docker example include the authentication and encryption configuration required by secure defaults

Why

Recent engine features were documented in focused guides and the changelog, but the primary README, architecture overview, API summary, and documentation index did not consistently expose them. This made implemented capabilities difficult to discover and left one Docker example inconsistent with startup requirements.

Impact

New users and operators can now discover the current feature set from the main entry points, while generated OpenAPI and the changelog remain the exhaustive sources of truth.

Validation

• git diff --check
• verified every relative Markdown link in the changed files resolves locally
• verified every top-level docs/*.md guide is linked from docs/README.md