refactor(acp-example): snapshot goldens are JSONL, not pretty-printed .txt

The goldens now mirror the shape of the surfaces they capture — one compact
JSON record per line — matching the wire (NDJSON stdout) and disk (JSONL
session log) formats, renamed *.golden.jsonl. They stay grep/jq-able and
faithful to what the agent emits, where the prior pretty-printed .txt was a
reformatted representation. Both normalizers drop the 2-space indent; the
normalizer spec asserts the compact form. All 11 goldens regenerated; replay
remains deterministic (8/8 across runs).

examples/acp-agent/tests/acp.snapshot.ts

@@ -72,12 +72,12 @@ for (const scenario of SCENARIOS) {
       }
 
       await expect(normalizeStdout(result.rawStdout, ctx))
-        .toMatchFileSnapshot(join(dir, 'stdout.golden.txt'))
+        .toMatchFileSnapshot(join(dir, 'stdout.golden.jsonl'))
 
       if (scenario.hasModelTurn) {
         expect(result.sessionLog, 'a model scenario must persist a session log').toBeDefined()
         await expect(normalizeSessionLog(result.sessionLog as string, ctx))
-          .toMatchFileSnapshot(join(dir, 'session.golden.txt'))
+          .toMatchFileSnapshot(join(dir, 'session.golden.jsonl'))
       }
     })
   })
@@ -99,7 +99,7 @@ describe('snapshot fixtures', () => {
       const dir = join(SNAPSHOTS_DIR, name)
       expect(existsSync(join(dir, 'input.json')), `${name}/input.json`).toBe(true)
       expect(existsSync(join(dir, 'session.jsonl')), `${name}/session.jsonl`).toBe(true)
-      expect(existsSync(join(dir, 'stdout.golden.txt')), `${name}/stdout.golden.txt`).toBe(true)
+      expect(existsSync(join(dir, 'stdout.golden.jsonl')), `${name}/stdout.golden.jsonl`).toBe(true)
     }
   })
 })