refactor(read-file): resume large reads by same-path offset only

Drop the dual footer and live-cursor reuse so continuation is a single stateless path+offset notice, matching the single-way resume direction. Pre-existing cursor tests become same-URI+offset chains; the truncation 10k exemption keys on the offset footer only. Paging mechanics untouched.

Author: TheGreatAxios

src/plugins/read-file-guard-plugin.test.ts

@@ -389,41 +389,34 @@ describe("CL-8979 large-file pagination", () => {
     );
   }, 120_000);
 
-  test("chains cursor resumption on a blob past the scan ceiling without re-scanning", async () => {
+  test("chains same-URI+offset continuation on a blob past the scan ceiling without re-scanning", async () => {
     const rows = Array.from({ length: BIG_LINES }, (_, i) => bigRow(i));
     const bytes = new TextEncoder().encode(`${rows.join("\n")}\n`);
     const run = blobChainRunner(async (key) => {
       if (key === "cl8979-blob") return bytes;
       throw new Error(`missing ${key}`);
     });
     const collected: string[] = [];
-    let path = "tool-output:///cl8979-blob";
+    const path = "tool-output:///cl8979-blob";
+    let offset = 0;
     let hops = 0;
     let sawOffsetFooter = false;
-    let sawCursorAlias = false;
     for (;;) {
-      const result = await run(`blob-${hops}`, { path, limit: 200 });
+      const result = await run(`blob-${hops}`, { path, limit: 200, offset });
       hops += 1;
       const content = String(result.content);
       expect(result.isError).toBeFalsy();
       expect(content).not.toContain("scan limit");
       collected.push(...bodyRows(content));
-      const cursor = /Use path="(tool-output:\/\/\/[^"]+)"/.exec(content);
-      const offset = continueOffset(content);
-      if (offset !== null) {
-        sawOffsetFooter = true;
-        path = "tool-output:///cl8979-blob";
-      }
-      if (cursor !== null) {
-        sawCursorAlias = true;
-        path = cursor[1] as string;
-      }
-      if (offset === null && cursor === null) break;
+      const next = continueOffset(content);
+      if (next === null) break;
+      sawOffsetFooter = true;
+      expect(next).toBeGreaterThan(offset);
+      offset = next;
       expect(hops).toBeLessThan(2000);
     }
     expect(hops).toBeGreaterThan(1);
     expect(sawOffsetFooter).toBe(true);
-    expect(sawCursorAlias).toBe(true);
     expect(collected.length).toBe(BIG_LINES);
     expect(collected[BIG_LINES - 1]).toBe(bigRow(BIG_LINES - 1));
   }, 120_000);

src/plugins/read-file-guard-plugin.ts

@@ -30,10 +30,8 @@ export const READ_FILE_MAX_SCAN_BYTES = 8 * 1024 * 1024;
 /** Refuse tool-output blobs larger than this before bounded paging. */
 export const READ_FILE_MAX_TOOL_OUTPUT_BYTES = READ_FILE_MAX_SCAN_BYTES;
 // Headroom reserved out of the byte budget for the continuation notice, so the
-// returned payload including the notice stays under READ_FILE_MAX_BYTES. Sized
-// for a dual footer: the plain `Use offset=` continuation plus the appended
-// single-use cursor alias on byte/scan-limit pages.
-const NOTICE_RESERVE_BYTES = 384;
+// returned payload including the notice stays under READ_FILE_MAX_BYTES.
+const NOTICE_RESERVE_BYTES = 256;
 
 const LINE_TRUNC_SUFFIX = ` ... [line truncated at ${READ_FILE_MAX_LINE_LENGTH} chars; full line remains in the file — use grep to match within the line]`;
 const TOOL_OUTPUT_CHUNK_BYTES = 64 * 1024;

src/plugins/result-truncation-plugin.ts

@@ -294,12 +294,11 @@ export async function applyToolResultTruncation(
 }
 
 // A read_file page already carries its own continuation contract (a plain
-// `Use offset=` footer, a single-use cursor alias, or both). Re-cutting it at
-// the 10k leisure cap would slice the footer off the page boundary and strand
-// the pagination chain, so footer-bearing read_file pages pass through intact.
+// `Use offset=` footer). Re-cutting it at the 10k leisure cap would slice the
+// footer off the page boundary and strand the pagination chain, so
+// footer-bearing read_file pages pass through intact.
 // Pages without a footer take the normal path.
-const READ_FILE_CONTINUATION_RE =
-  /Use offset=\d+ to continue\.|Use path="tool-output:\/\/\//;
+const READ_FILE_CONTINUATION_RE = /Use offset=\d+ to continue\./;
 
 function isPagedReadFilePage(
   toolName: string | undefined,