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>

.env.example

@@ -19,8 +19,8 @@
 # PROBOD_BASE_URL=http://localhost:8080
 # PROBOD_API_ADDR=:8080
 # PROBOD_API_CORS_ALLOWED_ORIGINS=http://localhost:8080,http://localhost:5173,http://localhost:5174
-# PROBOD_TRUST_CENTER_HTTP_ADDR=:80
-# PROBOD_TRUST_CENTER_HTTPS_ADDR=:443
+# PROBOD_TRUST_CENTER_HTTP_ADDR=:10080
+# PROBOD_TRUST_CENTER_HTTPS_ADDR=:10443
 
 # ── Observability ─────────────────────────────────────────────────────
 # PROBOD_METRICS_ADDR=localhost:8081