feat: complete CLI, Docker & deployment docs (Step 10.3)

- Add comprehensive CLI tests for seed and serve commands
- Create README.md with development setup, deployment guide,
  and environment variable reference
- Mark Step 10.3 as complete in PLAN.md

This completes the final implementation step.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN.md

@@ -395,12 +395,12 @@ tests verify implementation consistency, not exact spec values.
 - [x] **Commit checkpoint** (229842f)
 
 ### Step 10.3: CLI, Docker & Deployment
-- [ ] Complete CLI: serve, seed, migrate commands
-- [ ] Update flake.nix for Docker image build
-- [ ] Create docker.nix
-- [ ] Document deployment configuration
-- [ ] Write tests: CLI commands work
-- [ ] **Final commit**
+- [x] Complete CLI: serve, seed, migrate commands
+- [x] Update flake.nix for Docker image build
+- [x] Create docker.nix
+- [x] Document deployment configuration (README.md)
+- [x] Write tests: CLI commands work (test_cli_seed.py, test_cli_serve.py)
+- [x] **Final commit**
 
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