feat(hooks): add read-image-resizer hook

Intercepts Read tool output with image attachments and resizes to comply with Anthropic API limits (≤1568px long edge, ≤5MB). Only activates for Anthropic provider sessions and appends resize metadata (original/new resolution, token count) to tool output.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/read-image-resizer/image-resizer.test.ts

@@ -0,0 +1,132 @@
+/// <reference types="bun-types" />
+
+import { afterEach, describe, expect, it, mock } from "bun:test"
+
+const PNG_1X1_DATA_URL =
+  "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
+
+type ImageResizerModule = typeof import("./image-resizer")
+
+async function importFreshImageResizerModule(): Promise<ImageResizerModule> {
+  return import(`./image-resizer?test-${Date.now()}-${Math.random()}`)
+}
+
+describe("calculateTargetDimensions", () => {
+  it("returns null when dimensions are already within limits", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(800, 600)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null at exact long-edge boundary", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(1568, 1000)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("scales landscape dimensions by max long edge", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(3000, 2000)
+
+    //#then
+    expect(result).toEqual({
+      width: 1568,
+      height: Math.floor(2000 * (1568 / 3000)),
+    })
+  })
+
+  it("scales portrait dimensions by max long edge", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(2000, 3000)
+
+    //#then
+    expect(result).toEqual({
+      width: Math.floor(2000 * (1568 / 3000)),
+      height: 1568,
+    })
+  })
+
+  it("scales square dimensions to exact target", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(4000, 4000)
+
+    //#then
+    expect(result).toEqual({ width: 1568, height: 1568 })
+  })
+
+  it("uses custom maxLongEdge when provided", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(2000, 1000, 1000)
+
+    //#then
+    expect(result).toEqual({ width: 1000, height: 500 })
+  })
+})
+
+describe("resizeImage", () => {
+  afterEach(() => {
+    mock.restore()
+  })
+
+  it("returns null when sharp import fails", async () => {
+    //#given
+    mock.module("sharp", () => {
+      throw new Error("sharp unavailable")
+    })
+    const { resizeImage } = await importFreshImageResizerModule()
+
+    //#when
+    const result = await resizeImage(PNG_1X1_DATA_URL, "image/png", {
+      width: 1,
+      height: 1,
+    })
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null when sharp throws during resize", async () => {
+    //#given
+    const mockSharpFactory = mock(() => ({
+      resize: () => {
+        throw new Error("resize failed")
+      },
+    }))
+
+    mock.module("sharp", () => ({
+      default: mockSharpFactory,
+    }))
+    const { resizeImage } = await importFreshImageResizerModule()
+
+    //#when
+    const result = await resizeImage(PNG_1X1_DATA_URL, "image/png", {
+      width: 1,
+      height: 1,
+    })
+
+    //#then
+    expect(result).toBeNull()
+  })
+})