docs: add initial README and OpenSpec proposal

Add comprehensive README.md documenting project purpose, architecture,
tech stack, and development workflow. Includes OpenSpec proposal
(add-initial-readme) with project documentation specification.

The README describes current state only (early development phase) and
serves as entry point for developers and AI assistants.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

openspec/changes/add-initial-readme/specs/project-documentation/spec.md

@@ -0,0 +1,34 @@
+# Project Documentation Specification
+
+## ADDED Requirements
+
+### Requirement: README File
+The project root SHALL contain a README.md file that serves as the primary entry point for developers and AI assistants.
+
+#### Scenario: Repository discovery
+- **WHEN** a developer or AI assistant discovers the repository
+- **THEN** they can read README.md to understand project purpose and current state
+
+### Requirement: Current State Documentation
+The README.md SHALL accurately describe only the current implementation state, not planned features.
+
+#### Scenario: Accurate project status
+- **WHEN** reading the README.md
+- **THEN** it reflects what is actually implemented
+- **AND** does not promise or describe unimplemented features
+
+### Requirement: Getting Started Guide
+The README.md SHALL include instructions for contributors to get started with development.
+
+#### Scenario: New contributor onboarding
+- **WHEN** a new contributor wants to start developing
+- **THEN** README.md provides clear setup steps
+- **AND** explains the OpenSpec-based workflow
+
+### Requirement: OpenSpec Reference
+The README.md SHALL reference the OpenSpec documentation structure for detailed specifications.
+
+#### Scenario: Finding detailed specs
+- **WHEN** a developer needs detailed technical specifications
+- **THEN** README.md directs them to the `openspec/` directory
+- **AND** explains how to read and create proposals