feat(sessions): sessionForkFrom - branch conversations from a common ancestor (#33)

- task schema: optional sessionForkFrom (source session name); requires
  session target; self-fork rejected
- runner: resolves source newest .jsonl (fail 4 if none/outside dataRoot);
  passes MOSAIC_SESSION_FORK + MOSAIC_SESSION_DIR; result records lineage
- pi adapter: --fork <source> --session-dir <target> when forking;
  ephemeral default unchanged; plain session resume unchanged
- compose passthrough; RELEASE -> 0.0.7 (adapter changed)
- suite +9 cases (58 total): plumbing via mock stderr, validation
  negatives, live fork - child recalls ancestor code word, ancestor
  session file untouched

Closes #33

scripts/mosaic-task.mjs

@@ -129,7 +129,7 @@ function validateMission(document, file) {
 
 function validateTask(document, file) {
   if (!isPlainObject(document)) fail(2, "task must be a JSON object");
-  rejectUnknownKeys(document, ["taskVersion", "id", "prompt", "mission", "expectExact", "timeoutSeconds", "workspace", "capabilities", "session"], "task");
+  rejectUnknownKeys(document, ["taskVersion", "id", "prompt", "mission", "expectExact", "timeoutSeconds", "workspace", "capabilities", "session", "sessionForkFrom"], "task");
   if (document.taskVersion !== 1) {
     fail(2, `unsupported taskVersion: ${JSON.stringify(document.taskVersion)} (supported: 1)`);
   }
@@ -192,6 +192,23 @@ function validateTask(document, file) {
     session = document.session;
   }
 
+  // Session fork (M11): optional source session whose newest session file
+  // is branched (pi --fork) into the target session dir. Requires session.
+  let sessionForkFrom = null;
+  if (document.sessionForkFrom !== undefined && document.sessionForkFrom !== null) {
+    if (typeof document.sessionForkFrom !== "string" || document.sessionForkFrom.length === 0) {
+      fail(2, 'task "sessionForkFrom" must be a non-empty string when present');
+    }
+    validateId(document.sessionForkFrom, "task sessionForkFrom");
+    if (!session) {
+      fail(2, 'task "sessionForkFrom" requires "session" (the fork target) to be set');
+    }
+    if (document.sessionForkFrom === session) {
+      fail(2, 'task "sessionForkFrom" must differ from "session" (cannot fork onto itself)');
+    }
+    sessionForkFrom = document.sessionForkFrom;
+  }
+
   // Capabilities (M5): optional tools allowlist mapped by adapters to their
   // native permission flags. Absent = no tools.
   let tools = null;
@@ -224,6 +241,7 @@ function validateTask(document, file) {
     workspace,
     tools,
     session,
+    sessionForkFrom,
   };
 }
 
@@ -338,6 +356,26 @@ function runTask(taskFile, options = {}) {
     spawnEnv.MOSAIC_SESSION_DIR = `/var/lib/mosaic/sessions/${task.session}`;
   }
 
+  // Session fork (M11): resolve the source session's newest file; pi --fork
+  // branches it into the target dir without modifying the ancestor.
+  if (task.sessionForkFrom) {
+    const sourceDir = path.join(resolved.dataRoot, "sessions", task.sessionForkFrom);
+    let sources = [];
+    try {
+      sources = fs.readdirSync(sourceDir).filter((f) => f.endsWith(".jsonl")).sort();
+    } catch {
+      fail(4, `cannot fork: source session dir not found: ${sourceDir}`);
+    }
+    if (sources.length === 0) {
+      fail(4, `cannot fork: source session '${task.sessionForkFrom}' has no session files`);
+    }
+    const relative = path.relative(resolved.dataRoot, path.join(sourceDir, sources[sources.length - 1]));
+    if (relative.startsWith("..") || path.isAbsolute(relative)) {
+      fail(4, `source session is outside the configured dataRoot: ${sourceDir}`);
+    }
+    spawnEnv.MOSAIC_SESSION_FORK = `/var/lib/mosaic/${relative.split(path.sep).join("/")}`;
+  }
+
   const proc = spawnSync(
     "docker",
     ["compose", "run", "--rm", "-T", "mosaic-agent", task.prompt],
@@ -388,6 +426,7 @@ function runTask(taskFile, options = {}) {
     workspace: task.workspace,
     tools: effectiveTools,
     session: task.session,
+    sessionForkFrom: task.sessionForkFrom,
     exitCode: proc.status,
     signal: proc.signal ?? null,
     provider: resolved.execution.provider,