Add configuration validation

[sauravpanda]

Summary

• add bonsai config check for global and repository config layers
• report source-aware syntax, type, range, regex, and unknown-key diagnostics
• show merged effective configuration when valid
• add stable --json output and non-zero exits for validation errors
• include config source paths in ordinary load failures

Validation

• go mod verify
• go test -race ./...
• go test ./...
• go vet ./...
• go build ./...
• exercised valid defaults and invalid human/JSON CLI output

Closes #62