feat(cli): add --model option to run command for model override

Add -m, --model <provider/model> option to oh-my-opencode run command.
Allows users to override the model while keeping the agent unchanged.

Changes:
- Add model?: string to RunOptions interface
- Create model-resolver.ts to parse provider/model format
- Add model-resolver.test.ts with 7 test cases (TDD)
- Add --model CLI option with help text examples
- Wire resolveRunModel in runner.ts and pass to promptAsync
- Export resolveRunModel from barrel (index.ts)

Example usage:
  bunx oh-my-opencode run --model anthropic/claude-sonnet-4 "Fix the bug"
  bunx oh-my-opencode run --agent Sisyphus --model openai/gpt-5.4 "Task"

src/cli/run/model-resolver.ts

@@ -0,0 +1,29 @@
+export function resolveRunModel(
+  modelString?: string
+): { providerID: string; modelID: string } | undefined {
+  if (modelString === undefined) {
+    return undefined
+  }
+
+  const trimmed = modelString.trim()
+  if (trimmed.length === 0) {
+    throw new Error("Model string cannot be empty")
+  }
+
+  const parts = trimmed.split("/")
+  if (parts.length < 2) {
+    throw new Error("Model string must be in 'provider/model' format")
+  }
+
+  const providerID = parts[0]
+  if (providerID.length === 0) {
+    throw new Error("Provider cannot be empty")
+  }
+
+  const modelID = parts.slice(1).join("/")
+  if (modelID.length === 0) {
+    throw new Error("Model ID cannot be empty")
+  }
+
+  return { providerID, modelID }
+}