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/saml_config.go

@@ -24,7 +24,7 @@ type SAMLConfig struct {
 	Certificate                       string `json:"certificate"`
 	PrivateKey                        string `json:"private-key"`
 	DomainVerificationIntervalSeconds int    `json:"domain-verification-interval-seconds"`
-	DomainVerificationResolverAddr    string `json:"domain-verification-resolver-addr"`
+	DomainVerificationResolverAddr    string `json:"domain-verification-resolver-addr,omitempty"`
 }
 
 func (c SAMLConfig) SessionDurationTime() time.Duration {