feat(config): add config-as-code YAML import/export

Add declarative config-as-code support via YAML files. Monitors and
alerts can be exported, version controlled, and applied across instances.

- goupkeep export [-o file.yaml] dumps current state
- goupkeep apply -f file.yaml creates/updates to match desired state
- --dry-run shows planned changes without applying
- --prune deletes monitors/alerts not in the YAML
- Matching by name, alert references by name, nested group children
- CLI refactored to subcommands (apply, export, serve) with backward compat
- 24 tests covering apply, export, validation, round-trip idempotency

internal/config/types.go

@@ -0,0 +1,34 @@
+package config
+
+type File struct {
+	Alerts   []Alert   `yaml:"alerts,omitempty"`
+	Monitors []Monitor `yaml:"monitors,omitempty"`
+}
+
+type Alert struct {
+	Name     string            `yaml:"name"`
+	Type     string            `yaml:"type"`
+	Settings map[string]string `yaml:"settings"`
+}
+
+type Monitor struct {
+	Name            string    `yaml:"name"`
+	Type            string    `yaml:"type"`
+	URL             string    `yaml:"url,omitempty"`
+	Interval        int       `yaml:"interval,omitempty"`
+	Alert           string    `yaml:"alert,omitempty"`
+	CheckSSL        bool      `yaml:"check_ssl,omitempty"`
+	ExpiryThreshold int       `yaml:"expiry_threshold,omitempty"`
+	MaxRetries      int       `yaml:"max_retries,omitempty"`
+	Hostname        string    `yaml:"hostname,omitempty"`
+	Port            int       `yaml:"port,omitempty"`
+	Timeout         int       `yaml:"timeout,omitempty"`
+	Method          string    `yaml:"method,omitempty"`
+	Description     string    `yaml:"description,omitempty"`
+	AcceptedCodes   string    `yaml:"accepted_codes,omitempty"`
+	DNSResolveType  string    `yaml:"dns_resolve_type,omitempty"`
+	DNSServer       string    `yaml:"dns_server,omitempty"`
+	IgnoreTLS       bool      `yaml:"ignore_tls,omitempty"`
+	Paused          bool      `yaml:"paused,omitempty"`
+	Monitors        []Monitor `yaml:"monitors,omitempty"`
+}