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/bootstrap/builder_test.go

@@ -214,6 +214,7 @@ func TestBuilder_Build_Defaults(t *testing.T) {
 	assert.Equal(t, "gpt-4o", cfg.Probod.Agents.Default.ModelName)
 	assert.Equal(t, new(0.1), cfg.Probod.Agents.Default.Temperature)
 	assert.Equal(t, new(4096), cfg.Probod.Agents.Default.MaxTokens)
+	assert.Nil(t, cfg.Probod.Agents.Providers)
 	// Agents config — per-agent overrides are empty (inherit from default)
 	assert.Empty(t, cfg.Probod.Agents.Probo.Provider)
 	assert.Empty(t, cfg.Probod.Agents.Probo.ModelName)