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.test.ts

@@ -0,0 +1,83 @@
+/// <reference types="bun-types" />
+
+import { describe, it, expect } from "bun:test"
+import { resolveRunModel } from "./model-resolver"
+
+describe("resolveRunModel", () => {
+  it("given no model string, when resolved, then returns undefined", () => {
+    // given
+    const modelString = undefined
+
+    // when
+    const result = resolveRunModel(modelString)
+
+    // then
+    expect(result).toBeUndefined()
+  })
+
+  it("given empty string, when resolved, then throws Error", () => {
+    // given
+    const modelString = ""
+
+    // when
+    const resolve = () => resolveRunModel(modelString)
+
+    // then
+    expect(resolve).toThrow()
+  })
+
+  it("given valid 'anthropic/claude-sonnet-4', when resolved, then returns correct object", () => {
+    // given
+    const modelString = "anthropic/claude-sonnet-4"
+
+    // when
+    const result = resolveRunModel(modelString)
+
+    // then
+    expect(result).toEqual({ providerID: "anthropic", modelID: "claude-sonnet-4" })
+  })
+
+  it("given nested slashes 'openai/gpt-5.3/preview', when resolved, then modelID is 'gpt-5.3/preview'", () => {
+    // given
+    const modelString = "openai/gpt-5.3/preview"
+
+    // when
+    const result = resolveRunModel(modelString)
+
+    // then
+    expect(result).toEqual({ providerID: "openai", modelID: "gpt-5.3/preview" })
+  })
+
+  it("given no slash 'claude-sonnet-4', when resolved, then throws Error", () => {
+    // given
+    const modelString = "claude-sonnet-4"
+
+    // when
+    const resolve = () => resolveRunModel(modelString)
+
+    // then
+    expect(resolve).toThrow()
+  })
+
+  it("given empty provider '/claude-sonnet-4', when resolved, then throws Error", () => {
+    // given
+    const modelString = "/claude-sonnet-4"
+
+    // when
+    const resolve = () => resolveRunModel(modelString)
+
+    // then
+    expect(resolve).toThrow()
+  })
+
+  it("given trailing slash 'anthropic/', when resolved, then throws Error", () => {
+    // given
+    const modelString = "anthropic/"
+
+    // when
+    const resolve = () => resolveRunModel(modelString)
+
+    // then
+    expect(resolve).toThrow()
+  })
+})