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/aws_config.go

@@ -15,10 +15,10 @@
 package probodconfig
 
 type AWSConfig struct {
-	Region          string `json:"region"`
-	Bucket          string `json:"bucket"`
-	AccessKeyID     string `json:"access-key-id"`
-	SecretAccessKey string `json:"secret-access-key"`
-	Endpoint        string `json:"endpoint"`
+	Region          string `json:"region,omitempty"`
+	Bucket          string `json:"bucket,omitempty"`
+	AccessKeyID     string `json:"access-key-id,omitempty"`
+	SecretAccessKey string `json:"secret-access-key,omitempty"`
+	Endpoint        string `json:"endpoint,omitempty"`
 	UsePathStyle    bool   `json:"use-path-style"`
 }