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/oidc_config.go

@@ -15,7 +15,11 @@
 package probodconfig
 
 type OIDCProviderConfig struct {
-	ClientID     string `json:"client-id"`
-	ClientSecret string `json:"client-secret"`
-	Enabled      bool   `json:"enabled"`
+	ClientID     string `json:"client-id,omitempty"`
+	ClientSecret string `json:"client-secret,omitempty"`
+	Enabled      bool   `json:"enabled,omitempty"`
+}
+
+func (c OIDCProviderConfig) IsZero() bool {
+	return c.ClientID == "" && c.ClientSecret == ""
 }