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/index.ts

@@ -1,5 +1,6 @@
 export { run } from "./runner"
 export { resolveRunAgent } from "./agent-resolver"
+export { resolveRunModel } from "./model-resolver"
 export { createServerConnection } from "./server-connection"
 export { resolveSession } from "./session-resolver"
 export { createJsonOutputManager } from "./json-output"