Omit empty fields from bootstrap config output

probod-bootstrap was writing empty strings and stub blocks such as
`esign: {}` into generated YAML. The post-marshal prune pass caused
part of that by stripping empty leaf strings while leaving empty
parent maps behind.

Drop the prune round-trip in WriteConfig and rely on struct-level
omitzero/omitempty tags plus custom IsZero() helpers on probodconfig.
Only include LLM providers when an API key is set, use a nil map for
extra API headers, and extend the dev-config Makefile recipe with the
local dev defaults already documented in .env.example.

Config loading is unchanged: omitted keys still decode to Go zero
values.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/probodconfig/custom_domains_config.go

@@ -17,16 +17,16 @@ package probodconfig
 type CustomDomainsConfig struct {
 	RenewalInterval   int        `json:"renewal-interval"`
 	ProvisionInterval int        `json:"provision-interval"`
-	ResolverAddr      string     `json:"resolver-addr"`
+	ResolverAddr      string     `json:"resolver-addr,omitempty"`
 	CnameTarget       string     `json:"cname-target"`
 	CAAIssuerDomain   string     `json:"caa-issuer-domain"`
-	ACME              ACMEConfig `json:"acme"`
+	ACME              ACMEConfig `json:"acme,omitzero"`
 }
 
 type ACMEConfig struct {
-	Directory  string `json:"directory"`
-	Email      string `json:"email"`
-	KeyType    string `json:"key-type"`
-	AccountKey string `json:"account-key"`
-	RootCA     string `json:"root-ca"`
+	Directory  string `json:"directory,omitempty"`
+	Email      string `json:"email,omitempty"`
+	KeyType    string `json:"key-type,omitempty"`
+	AccountKey string `json:"account-key,omitempty"`
+	RootCA     string `json:"root-ca,omitempty"`
 }