Add prefix/suffix support, short ID fallback, unit docs

- config: add Prefix/Suffix fields to Config struct
- systemd: ServiceName/Generate/UnitPath/Install/Uninstall/Enable/Disable/Status all accept prefix+suffix
- runtime: fall back to short container ID (12 chars) when container has no name
- cmd: active, status, install all thread prefix/suffix from config
- systemd/generator_test.go: updated all calls + added TestGenerate_WithPrefixSuffix
- docs/generated-units.md: full examples of every unit type + ordering + naming
- README: updated config docs, prefix/suffix section, link to docs/

config/config.go

@@ -24,7 +24,9 @@ type Unit struct {
 
 // Config is the root config structure.
 type Config struct {
-	Units []Unit `yaml:"units"`
+	Units  []Unit `yaml:"units"`
+	Prefix string `yaml:"prefix,omitempty"` // prepended to generated service name, e.g. "prod-"
+	Suffix string `yaml:"suffix,omitempty"` // appended to generated service name, e.g. "-svc"
 }
 
 // Load reads and parses the config file at path.