release: v1.4.0 — release notes, version bump, repo hygiene

- Add assets/releases/ver1-4-0.md covering deltas since v1.4.0-beta
  (reasoning_effort normalization, tool-schema fallback, restart-safe
  turn runtime, frontend placeholder hardening, CLI chat REPL config).
- Bump deeptutor/__version__ to 1.4.0.
- Update README Releases section: v1.4.0 at top, install opt-in note
  updated for stable.
- Fix pre-existing pre-commit failures so the v1.4.0 cut runs clean:
  bandit B324 (mark _sha1 as non-security), mypy no-redef in
  document.py / line_doc.py, mypy arg-type/assignment in
  snapshot/adapters.py, mypy assignment + arg-type in
  services/memory/settings.py, mypy func-returns-value on
  set.add idiom in deeptutor_cli/init_wizard.py.
- Apply ruff format / prettier sweep across the repo.
- Remove stale workspace + i18n env example + playwright captures
  already cleaned locally; ignore .playwright-cli/ going forward.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>