test: add operational-learning E2E test (gate-tier)

Validates the write path: agent encounters a CLI failure, logs an
operational learning to JSONL via gstack-learnings-log. Replaces the
removed contributor-mode E2E test.

Setup: temp git repo, copy bin scripts, set GSTACK_HOME.
Prompt: simulated npm test failure needing --experimental-vm-modules.
Assert: learnings.jsonl exists with type=operational entry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/helpers/touchfiles.ts

@@ -42,6 +42,7 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   'skillmd-outside-git':      ['SKILL.md', 'SKILL.md.tmpl', 'scripts/gen-skill-docs.ts'],
 
   'session-awareness':        ['SKILL.md', 'SKILL.md.tmpl', 'scripts/gen-skill-docs.ts'],
+  'operational-learning':     ['scripts/resolvers/preamble.ts', 'bin/gstack-learnings-log'],
 
   // QA (+ test-server dependency)
   'qa-quick':       ['qa/**', 'browse/src/**', 'browse/test/test-server.ts'],
@@ -182,6 +183,7 @@ export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
   'skillmd-no-local-binary': 'gate',
   'skillmd-outside-git': 'gate',
   'session-awareness': 'gate',
+  'operational-learning': 'gate',
 
   // QA — gate for functional, periodic for quality/benchmarks
   'qa-quick': 'gate',