fix(jsonl): warn when repairing torn tails

packages/session/session-persistence-jsonl/src/index.ts

@@ -441,6 +441,7 @@ export class JsonlSessionPersistence extends SessionPersistence implements Persi
     if (tornMarker !== undefined) await this.repair(meta, tornMarker.truncateTo)
     const repairedEvents = [...(tornMarker?.recoveredEvents ?? []), ...closers]
     if (repairedEvents.length > 0) await this.appendLines(meta, repairedEvents)
+    if (tornMarker !== undefined) this.ctx.logger.warn(`${this.name}: session "${meta.id}" recovered from a torn tail; incomplete tail bytes were discarded`)
   }
 
   /** List valid unique stored sessions' metadata (header line only — no full-log parse). */

packages/session/session-persistence-jsonl/tests/zstd.spec.ts

@@ -539,6 +539,7 @@ describe('JsonlSessionPersistence: default Zstandard encoding', () => {
     const root = await freshRoot()
     const ctx = await mount(root)
     const header = meta('recover-torn', '/proj')
+    const warn = vi.spyOn(ctx.logger, 'warn').mockImplementation(() => undefined)
     await ctx.sessionPersistence.create(header)
     await ctx.sessionPersistence.append(header.id, oneTurnLog())
     const path = logPath(root, header.cwd, header.id, 'zstd')
@@ -562,6 +563,7 @@ describe('JsonlSessionPersistence: default Zstandard encoding', () => {
     expect(loaded.events.some(event => event.type === 'assistant/chunk' && event.seq === 8)).toBe(false)
     expect(loaded.events[8]?.type).toBe('step/end')
     expect(loaded.events[9]?.type).toBe('turn/end')
+    expect(warn).toHaveBeenCalledWith('session-persistence-jsonl: session "recover-torn" recovered from a torn tail; incomplete tail bytes were discarded')
 
     const repaired = await readFile(path)
     expect(repaired.subarray(0, committed.length)).toEqual(committed)