perf(hashline): use write metadata line counts

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

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

src/hooks/hashline-read-enhancer/hook.ts

@@ -141,6 +141,22 @@ function extractFilePath(metadata: unknown): string | undefined {
   return undefined
 }
 
+function extractLineCount(metadata: unknown): number | undefined {
+  if (!metadata || typeof metadata !== "object") {
+    return undefined
+  }
+
+  const objectMeta = metadata as Record<string, unknown>
+  const candidates = [objectMeta.lineCount, objectMeta.linesWritten, objectMeta.lines]
+  for (const candidate of candidates) {
+    if (typeof candidate === "number" && Number.isInteger(candidate) && candidate >= 0) {
+      return candidate
+    }
+  }
+
+  return undefined
+}
+
 async function appendWriteHashlineOutput(output: { output: string; metadata: unknown }): Promise<void> {
   if (output.output.startsWith(WRITE_SUCCESS_MARKER)) {
     return
@@ -151,6 +167,12 @@ async function appendWriteHashlineOutput(output: { output: string; metadata: unk
     return
   }
 
+  const metadataLineCount = extractLineCount(output.metadata)
+  if (metadataLineCount !== undefined) {
+    output.output = `${WRITE_SUCCESS_MARKER} ${metadataLineCount} lines written.`
+    return
+  }
+
   const filePath = extractFilePath(output.metadata)
   if (!filePath) {
     return

src/hooks/hashline-read-enhancer/index.test.ts

@@ -11,9 +11,9 @@ function mockCtx(): PluginInput {
   return {
     client: {} as PluginInput["client"],
     directory: "/test",
-    project: "/test" as unknown as PluginInput["project"],
+    project: "/test" as PluginInput["project"],
     worktree: "/test",
-    serverUrl: "http://localhost" as unknown as PluginInput["serverUrl"],
+    serverUrl: "http://localhost" as PluginInput["serverUrl"],
     $: {} as PluginInput["$"],
   }
 }
@@ -238,6 +238,26 @@ describe("hashline-read-enhancer", () => {
     fs.rmSync(tempDir, { recursive: true, force: true })
   })
 
+  it("uses write metadata line count without reading the file", async () => {
+    //#given
+    const hook = createHashlineReadEnhancerHook(mockCtx(), { hashline_edit: { enabled: true } })
+    const input = { tool: "write", sessionID: "s", callID: "c" }
+    const output = {
+      title: "write",
+      output: "Wrote file successfully.",
+      metadata: {
+        filepath: "/tmp/hashline-metadata-fast-path-missing-file.ts",
+        lineCount: 7,
+      },
+    }
+
+    //#when
+    await hook["tool.execute.after"](input, output)
+
+    //#then
+    expect(output.output).toBe("File written successfully. 7 lines written.")
+  })
+
   it("does not overwrite write tool error output with success message", async () => {
     //#given — write tool failed, but stale file exists from previous write
     const hook = createHashlineReadEnhancerHook(mockCtx(), { hashline_edit: { enabled: true } })