feat: allow specifying model provider in preferences (#350)

Add explicit provider targeting for model preferences when the same
model ID exists across multiple providers (e.g., claude-sonnet-4-6
on both Anthropic and Bedrock).

Two formats supported:
- String: "bedrock/claude-sonnet-4-6"
- Object: { model: claude-sonnet-4-6, provider: bedrock }

The provider/model string format already worked in the resolution
code but was undocumented. This adds the provider field to the
object format and documents both approaches.

src/resources/extensions/gsd/docs/preferences-reference.md

@@ -84,7 +84,9 @@ Setting `prefer_skills: []` does **not** disable skill discovery — it just mea
 
 - `models`: per-stage model selection for auto-mode. Keys: `research`, `planning`, `execution`, `completion`. Values can be:
   - Simple string: `"claude-sonnet-4-6"` — single model, no fallbacks
+  - Provider-qualified string: `"bedrock/claude-sonnet-4-6"` — targets a specific provider when the same model ID exists across multiple providers
   - Object with fallbacks: `{ model: "claude-opus-4-6", fallbacks: ["glm-5", "minimax-m2.5"] }` — tries fallbacks in order if primary fails
+  - Object with provider: `{ model: "claude-opus-4-6", provider: "bedrock" }` — explicit provider targeting in object format
   - Omit a key to use whatever model is currently active. Fallbacks are tried when model switching fails (provider unavailable, rate limited, etc.).
 
 - `skill_discovery`: controls how GSD discovers and applies skills during auto-mode. Valid values:
@@ -181,6 +183,29 @@ models:
 
 When a model fails to switch (provider unavailable, rate limited, credits exhausted), GSD automatically tries the next model in the `fallbacks` list. This ensures auto-mode continues even when your preferred provider hits limits.
 
+## Provider Targeting
+
+When the same model ID exists across multiple providers (e.g., `claude-sonnet-4-6` on both Anthropic and Bedrock), use the `provider/model` format or the `provider` field to target a specific one:
+
+```yaml
+---
+version: 1
+models:
+  # String format: provider/model
+  research: bedrock/claude-sonnet-4-6
+  planning: anthropic/claude-opus-4-6
+
+  # Object format: explicit provider field
+  execution:
+    model: claude-sonnet-4-6
+    provider: bedrock
+    fallbacks:
+      - anthropic/claude-sonnet-4-6
+---
+```
+
+If you use a bare model ID (no provider prefix) and it exists in multiple providers, GSD will warn you and resolve to the first available match. Use `provider/model` format to avoid ambiguity.
+
 **Cost-optimized example** — use cheap models with expensive ones as fallback for critical phases:
 
 ```yaml

src/resources/extensions/gsd/preferences.ts

@@ -28,6 +28,8 @@ export interface GSDSkillRule {
 export interface GSDPhaseModelConfig {
   /** Primary model ID (e.g., "claude-opus-4-6") */
   model: string;
+  /** Provider name to disambiguate when the same model ID exists across providers (e.g., "bedrock", "anthropic") */
+  provider?: string;
   /** Fallback models to try in order if primary fails (e.g., rate limits, credits exhausted) */
   fallbacks?: string[];
 }
@@ -580,8 +582,14 @@ export function resolveModelWithFallbacksForUnit(unitType: string): ResolvedMode
     return { primary: phaseConfig, fallbacks: [] };
   }
 
+  // When provider is explicitly set, prepend it to the model ID so the
+  // resolution code in auto.ts can do an explicit provider match.
+  const primary = phaseConfig.provider && !phaseConfig.model.includes("/")
+    ? `${phaseConfig.provider}/${phaseConfig.model}`
+    : phaseConfig.model;
+
   return {
-    primary: phaseConfig.model,
+    primary,
     fallbacks: phaseConfig.fallbacks ?? [],
   };
 }