fix(retry): lineage tracking + relative mission path resolution (#28)

- retryRun rewrites a snapshot's relative mission path to the run's own
  recorded mission.json (absolute) before execution — retries stay
  faithful to what originally ran
- runTask accepts options.retriedFrom; retry records lineage in
  result.json (additive optional field, no schema break)
- task suite +4 cases: retry succeeds, lineage recorded, mission section
  present after retry (36 total), missing-run retry exits 4

Closes #28

scripts/mosaic-task.mjs

@@ -234,7 +234,7 @@ function writeOnce(file, content) {
   }
 }
 
-function runTask(taskFile) {
+function runTask(taskFile, options = {}) {
   const resolved = JSON.parse(
     spawnSync(process.execPath, [path.join(PROJECT_ROOT, "scripts", "mosaic-config.mjs"), "validate"], {
       cwd: PROJECT_ROOT,
@@ -345,6 +345,7 @@ function runTask(taskFile) {
     request: task.prompt,
     response,
     expectedExact: expected,
+    ...(options.retriedFrom ? { retriedFrom: options.retriedFrom } : {}),
     workspace: task.workspace,
     tools: task.tools,
     session: task.session,
@@ -455,6 +456,24 @@ function retryRun(runId) {
     fail(4, `run task snapshot unreadable: ${runId}`);
   }
 
+  // Relative mission paths in a snapshot resolve against the ORIGINAL task
+  // location, which no longer exists here — rewrite them to the run's own
+  // recorded mission.json so retries stay faithful.
+  let snapshotDoc;
+  try {
+    snapshotDoc = JSON.parse(snapshot);
+  } catch {
+    fail(4, `run task snapshot is not valid JSON: ${runId}`);
+  }
+  if (snapshotDoc.mission && !path.isAbsolute(snapshotDoc.mission)) {
+    const recordedMission = path.join(dir, "mission.json");
+    if (!fs.existsSync(recordedMission)) {
+      fail(4, `cannot retry ${runId}: relative mission path but no mission.json snapshot in run dir`);
+    }
+    snapshotDoc.mission = recordedMission;
+    snapshot = `${JSON.stringify(snapshotDoc, null, 2)}\n`;
+  }
+
   // A retry is a brand-new run: replay the recorded task snapshot through
   // the ordinary run path; existing run records stay untouched.
   const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "mosaic-retry-"));
@@ -467,7 +486,7 @@ function retryRun(runId) {
       // Best-effort cleanup only.
     }
   });
-  runTask(tempTaskFile);
+  runTask(tempTaskFile, { retriedFrom: runId });
 }
 
 const operation = process.argv[2];