feat(ultraplan-local): mirror Phase 2.6 hardenings in headless-launch-template

Bring the launch template (used by /ultraplan-local --decompose) into
contract-parity with the Phase 2.6 wave executor hardenings shipped in the
previous commit:

- GIT_OPTIONAL_LOCKS=0 exported once at the top
- MAX_TURNS / MAX_BUDGET_USD env-overridable (default 50 / 5)
- Absolute SHARED_CONTEXT_FILE built from brief + architecture
- SAFETY_PREAMBLE prepended to every per-session prompt (GH #36071 +
  GH #52272 clarifications)
- Per-child --max-turns + --max-budget-usd + --append-system-prompt-file
- push-before-cleanup before merge AND in the cleanup_worktrees trap
- Three new template rules (16, 17, 18, 19) document the contract for
  session-decomposer

Pin in tests/lib/doc-consistency.test.mjs locks all required substrings
against future regressions.

plugins/ultraplan-local/tests/lib/doc-consistency.test.mjs

@@ -158,6 +158,26 @@ test('rule-catalogue has exactly 12 entries', async () => {
   );
 });
 
+test('headless-launch-template.md mirrors Phase 2.6 hardenings', () => {
+  const tpl = read('templates/headless-launch-template.md');
+  for (const needle of [
+    'GIT_OPTIONAL_LOCKS',
+    '--max-turns',
+    '--max-budget-usd',
+    '--append-system-prompt-file',
+    'SHARED_CONTEXT_FILE',
+    'SAFETY_PREAMBLE',
+    'git push origin',
+    'GH #36071',
+    'push-before-cleanup',
+  ]) {
+    assert.ok(
+      tpl.includes(needle),
+      `templates/headless-launch-template.md should include "${needle}" (Step 10 mirrors Phase 2.6)`,
+    );
+  }
+});
+
 test('Phase 9 prose mandates parallel single-message dispatch + inline dedup', () => {
   const cmd = read('commands/ultraplan-local.md');
   const orch = read('agents/planning-orchestrator.md');