feat(sessions): named persistent sessions with resume (L1) (#22, #23)

- task schema: optional session (named id) -> persistent session dir at
  dataRoot/sessions/<name>, isolated per name
- pi adapter: --session-dir when declared (ephemeral --no-session stays
  the default otherwise); -c resumes the most recent session when present
- compose passthrough; result.json records session
- fixtures: tasks/session-demo-1.json (teach) + session-demo-2.json (recall)
- E2E: teach -> REMEMBERED + host-side session JSONL; resume -> recalled
  'mosaico' exactly; single continued session file

Closes #22, closes #23

scripts/mosaic-task.mjs

@@ -106,7 +106,7 @@ 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"], "task");
+  rejectUnknownKeys(document, ["taskVersion", "id", "prompt", "mission", "expectExact", "timeoutSeconds", "workspace", "capabilities", "session"], "task");
   if (document.taskVersion !== 1) {
     fail(2, `unsupported taskVersion: ${JSON.stringify(document.taskVersion)} (supported: 1)`);
   }
@@ -158,6 +158,17 @@ function validateTask(document, file) {
     workspace = document.workspace;
   }
 
+  // Session (M6): optional named persistent session under
+  // <dataRoot>/sessions/<name>. Distinct names never share state.
+  let session = null;
+  if (document.session !== undefined && document.session !== null) {
+    if (typeof document.session !== "string" || document.session.length === 0) {
+      fail(2, 'task "session" must be a non-empty string when present');
+    }
+    validateId(document.session, "task session");
+    session = document.session;
+  }
+
   // Capabilities (M5): optional tools allowlist mapped by adapters to their
   // native permission flags. Absent = no tools.
   let tools = null;
@@ -189,6 +200,7 @@ function validateTask(document, file) {
     timeoutSeconds,
     workspace,
     tools,
+    session,
   };
 }
 
@@ -271,6 +283,12 @@ function runTask(taskFile) {
   if (workspaceContainerPath) spawnEnv.MOSAIC_WORKSPACE = workspaceContainerPath;
   spawnEnv.MOSAIC_TOOLS = task.tools ? task.tools.join(",") : "";
 
+  // Session (M6): persistent named session dir, passed as container path.
+  if (task.session) {
+    fs.mkdirSync(path.join(resolved.dataRoot, "sessions", task.session), { recursive: true });
+    spawnEnv.MOSAIC_SESSION_DIR = `/var/lib/mosaic/sessions/${task.session}`;
+  }
+
   const proc = spawnSync(
     "docker",
     ["compose", "run", "--rm", "-T", "mosaic-agent", task.prompt],
@@ -319,6 +337,7 @@ function runTask(taskFile) {
     expectedExact: expected,
     workspace: task.workspace,
     tools: task.tools,
+    session: task.session,
     exitCode: proc.status,
     signal: proc.signal ?? null,
     provider: resolved.execution.provider,