feat(FN-2662): enforce model override fallback hierarchy

- Fix triage planning model resolution to fall back through project/global planning settings and default overrides
- Fix reviewer model selection to honor validator-specific settings before default provider/model overrides
- Update merger model resolution to apply default override fallback and align shared task setting types/executor flow
- Add regression coverage for triage, reviewer, and merger fallback behavior and update settings hierarchy documentation

docs/settings-reference.md

@@ -304,6 +304,12 @@ Fusion uses a dual-scope model settings system with five lanes. Global settings
 5. Global `defaultProvider` + `defaultModelId`
 6. Automatic provider/model resolution
 
+### Merger model
+
+1. Project `defaultProviderOverride` + `defaultModelIdOverride`
+2. Global `defaultProvider` + `defaultModelId`
+3. Automatic provider/model resolution
+
 ### Title summarization model
 
 1. Project `titleSummarizerProvider` + `titleSummarizerModelId`
@@ -313,7 +319,7 @@ Fusion uses a dual-scope model settings system with five lanes. Global settings
 5. Global `defaultProvider` + `defaultModelId`
 6. Automatic provider/model resolution
 
-> **Note:** Runtime fallback precedence logic is implemented in engine and dashboard routes (FN-1711). The hierarchy above reflects the full schema contracts added in FN-1710.
+> **Note:** Runtime fallback precedence logic is implemented in engine and dashboard routes. The hierarchies above reflect current runtime behavior.
 
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