update-feature-config

Add the field to the section struct in pkg/lib/config/feature_*.go, extend that section's Merge with a per-field nil check (if layer.X != nil { c.X = layer.X }), and add a layered test case in testdata/merge_feature.yaml proving sibling fields survive partial overrides.

Whole-section merge swaps (return layer.Section) discard sibling fields set by lower layers when a higher layer only sets one field. Plan and app YAML documents are routinely partial, so every multi-field section must merge field-by-field instead of replacing the whole section.

Add a case to pkg/lib/config/testdata/merge_feature.yaml where one layer sets field A only and a later layer sets sibling B only, then assert both survive. Use non-default values so a whole-section regression cannot accidentally produce the right result.

A whole-object replace is fine only when a section genuinely has one leaf field with no siblings at any deeper level. If the single field is itself an object containing siblings, the field-level merge must cascade down to where the real siblings live.

Scalar fields tagged omitempty are omitted by encoding/json when their zero value is the real default, so a fully resolved section marshals as {}. Drop omitempty on plain bool, string, and numeric fields and verify with a marshal-based test rather than struct comparison.

Only after checking how the runtime consumer treats edge-case values like nil, empty, or zero. A constraint stricter than runtime semantics can block meaningful inputs, such as an empty allowlist that the code already interprets as no restriction.