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/slack_config.go

@@ -16,5 +16,5 @@ package probodconfig
 
 type SlackConfig struct {
 	SenderInterval int    `json:"sender-interval"`
-	SigningSecret  string `json:"signing-secret"`
+	SigningSecret  string `json:"signing-secret,omitempty"`
 }