feat(hooks): surface fsync-skip warnings to AI agent via tool output

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

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

src/shared/fsync-skip-warning-formatter.test.ts

@@ -0,0 +1,78 @@
+import { describe, expect, it } from "bun:test"
+
+import type { FsyncSkipEntry } from "./fsync-skip-tracker"
+import { formatFsyncSkipWarning } from "./fsync-skip-warning-formatter"
+
+function makeEntry(index: number, classification: FsyncSkipEntry["pathClassification"]): FsyncSkipEntry {
+  return {
+    filePath: `/path/${index}`,
+    contextLabel: `atomicWrite:/path/${index}`,
+    errorCode: "EPERM",
+    message: "operation not permitted",
+    pathClassification: classification,
+    timestamp: 1000 + index,
+  }
+}
+
+describe("formatFsyncSkipWarning", () => {
+  it("returns empty string for zero entries", () => {
+    expect(formatFsyncSkipWarning([])).toBe("")
+  })
+
+  it("includes iCloud environment, path, and code for one entry", () => {
+    const warning = formatFsyncSkipWarning([makeEntry(1, "icloud")])
+    expect(warning).toContain("iCloud Drive")
+    expect(warning).toContain("/path/1")
+    expect(warning).toContain("EPERM")
+  })
+
+  it("shows all five paths when exactly five entries exist", () => {
+    const warning = formatFsyncSkipWarning([
+      makeEntry(1, "icloud"),
+      makeEntry(2, "icloud"),
+      makeEntry(3, "icloud"),
+      makeEntry(4, "icloud"),
+      makeEntry(5, "icloud"),
+    ])
+
+    expect(warning).toContain("/path/1")
+    expect(warning).toContain("/path/5")
+    expect(warning).not.toContain("and 1 more")
+  })
+
+  it("shows five paths plus overflow summary when six entries exist", () => {
+    const warning = formatFsyncSkipWarning([
+      makeEntry(1, "icloud"),
+      makeEntry(2, "icloud"),
+      makeEntry(3, "icloud"),
+      makeEntry(4, "icloud"),
+      makeEntry(5, "icloud"),
+      makeEntry(6, "icloud"),
+    ])
+
+    expect(warning).toContain("/path/5")
+    expect(warning).not.toContain("/path/6")
+    expect(warning).toContain("... and 1 more")
+  })
+
+  it("uses the most common classification when entries are mixed", () => {
+    const warning = formatFsyncSkipWarning([
+      makeEntry(1, "onedrive"),
+      makeEntry(2, "onedrive"),
+      makeEntry(3, "icloud"),
+    ])
+
+    expect(warning).toContain("Detected environment: OneDrive")
+  })
+
+  it("matches required section format", () => {
+    const warning = formatFsyncSkipWarning([makeEntry(1, "unknown")])
+
+    expect(warning).toContain("[fsync-skipped] 1 write(s) bypassed fsync")
+    expect(warning).toContain("Affected paths:")
+    expect(warning).toContain("What this means:")
+    expect(warning).toContain("The write+rename succeeded")
+    expect(warning).not.toContain("Detected environment:")
+    expect(warning).toContain("filesystem does not support fsync")
+  })
+})