feat: add integration testing infrastructure with test isolation

[TechDufus]

Summary

• Add OPENKANBAN_CONFIG_DIR env var support for test isolation (with XDG fallback)
• Create internal/testutil package with TestEnv helper for isolated test environments
• Add 5 integration tests for project/ticket lifecycle behind //go:build integration tag
• Add Makefile with targets: test, test-unit, test-integration, test-all, coverage, lint
• Update CI to run unit and integration tests in separate jobs
• Document testing patterns in AGENTS.md for AI agent awareness

Usage

make test              # Unit tests (default)
make test-integration  # Integration tests (isolated)
make test-all          # Both

# Manual testing without touching real data
OPENKANBAN_CONFIG_DIR=/tmp/test openkanban