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/mailer_config.go

@@ -16,15 +16,15 @@ package probodconfig
 
 type MailerConfig struct {
 	MailerInterval int        `json:"mailer-interval"`
-	SenderName     string     `json:"sender-name"`
-	SenderEmail    string     `json:"sender-email"`
-	SMTP           SMTPConfig `json:"smtp"`
+	SenderName     string     `json:"sender-name,omitempty"`
+	SenderEmail    string     `json:"sender-email,omitempty"`
+	SMTP           SMTPConfig `json:"smtp,omitzero"`
 }
 
 type SMTPConfig struct {
-	Addr        string `json:"addr"`
-	User        string `json:"user"`
-	Password    string `json:"password"`
+	Addr        string `json:"addr,omitempty"`
+	User        string `json:"user,omitempty"`
+	Password    string `json:"password,omitempty"`
 	TLSRequired bool   `json:"tls-required"`
-	HelloName   string `json:"hello-name"`
+	HelloName   string `json:"hello-name,omitempty"`
 }