Pre-release fixes and polish for open source launch

Fixed critical issues identified in code review:
- Fixed invalid Go version (1.25.2 → 1.21) in go.mod
- Fixed unchecked error in import.go JSON unmarshaling
- Fixed unchecked error returns in test cleanup (export_import_test.go, import_collision_test.go)
- Removed duplicate test code in dependencies_test.go via helper function

Added release infrastructure:
- Added 'bd version' command with JSON output support
- Created comprehensive CHANGELOG.md following Keep a Changelog format
- Updated README.md with clear alpha status warnings

All tests passing. Ready for public repository opening.

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

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

go.mod

@@ -1,6 +1,6 @@
 module github.com/steveyegge/beads
 
-go 1.25.2
+go 1.21
 
 require (
 	github.com/fatih/color v1.18.0 // indirect