chore: initialise Noema repository

Foundation for the persistent, inspectable cognitive runtime: README,
implementation plan with phases and risk register, architecture,
security model, threat model, contributing guide, eight ADRs covering
the global workspace, event immutability, memory representation, LLM
abstraction, capability system, authentication, frontend and storage.
Includes Makefile, env example, compose stack (pgvector) and Dockerfile
skeleton, plus a record of the development environment assumptions.

CHANGELOG.md

@@ -0,0 +1,8 @@
+# Changelog
+
+All notable changes to Noema are recorded here. Format follows Keep a Changelog; versions follow SemVer.
+
+## [Unreleased]
+
+### Added
+- Repository foundation: plan, architecture, security model, threat model, ADRs, Makefile, container files.