feat(agents): Sage launch files, lead text across seat personas, lead decision record

- agents/sage/ launch files committed after Dewey's review (R1 revise, R2
  approve). The launcher test now covers sage with its zai/glm-5.3 high pin.
  The README states the lead role and its limits, and the seat reads but
  never writes the old DYOR records under ~/.mosaic.
- N6 (Dewey authored, Sage reviewed against pins): seat personas and the
  Rocko launcher name Sage as project lead and Darkwing as a collaborating
  engineering seat, per Jason's 2026-09-26 ruling.
- docs/plans/2026-09-26_lead-decisions.md: the push, no merge into next and
  its conditions, the board restart, queue-as-data rulings, Gate F waiting
  on a T3 source, and what stays with Jason.

Launcher tests 6/6 and 1/1, eight suites green. Not pushed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

agents/sage/validate-sessions.mjs

@@ -0,0 +1,21 @@
+// Refuse damaged history before Pi's --continue can silently skip it.
+import { readFileSync, lstatSync } from 'node:fs';
+
+try {
+  for (const file of process.argv.slice(2)) {
+    if (!lstatSync(file).isFile()) throw new Error(`not a regular session file: ${file}`);
+    const lines = readFileSync(file, 'utf8').trim().split('\n');
+    const entries = lines.map((line) => JSON.parse(line));
+    const header = entries[0];
+    if (header?.type !== 'session' || typeof header.id !== 'string' || !header.id ||
+        typeof header.version !== 'number' || typeof header.cwd !== 'string' ||
+        !Number.isFinite(Date.parse(header.timestamp)) ||
+        entries.slice(1).some((entry) => !entry || typeof entry.type !== 'string')) {
+      throw new Error(`invalid session structure: ${file}`);
+    }
+    if (header.cwd !== process.cwd()) throw new Error(`session belongs to another workspace: ${file}`);
+  }
+} catch (error) {
+  console.error(`sage: cannot safely resume: ${error.message}; inspect history or explicitly use --fresh`);
+  process.exit(1);
+}