🤖 feat: implement Cobra CLI with subcommands

- Add new CLI structure in cmd/cli/
- Implement version, server, and greet commands
- Update build script to compile new CLI binary
- Add Cobra dependency to go.mod
- Update ADR 0015 to reflect implementation status
- Update README and AGENTS.md with CLI usage
- Maintain backward compatibility with existing binaries

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/go-playground/universal-translator v0.18.1
 	github.com/go-playground/validator/v10 v10.30.2
 	github.com/rs/zerolog v1.35.0
+	github.com/spf13/cobra v1.8.0
 	github.com/spf13/viper v1.21.0
 	github.com/swaggo/http-swagger v1.3.4
 	github.com/swaggo/swag v1.16.6
@@ -41,6 +42,7 @@ require (
 	github.com/hashicorp/go-immutable-radix v1.3.1 // indirect
 	github.com/hashicorp/go-memdb v1.3.5 // indirect
 	github.com/hashicorp/golang-lru v1.0.2 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mailru/easyjson v0.7.6 // indirect