docs: clarify UI verification for agents

[kocaemre]

Summary

Adds focused UI verification guidance for agent-assisted changes:

• expands AGENTS.md from a two-line Playwright note into a scoped UI verification playbook
• clarifies when Electron/UI smoke evidence is useful and when it is overkill
• adds a PR checklist item for UI verification evidence or an explicit no-UI-surface statement

Fixes #406.

Checklist

• I have read the Contributing guide.
• I have installed and tested the application for my platform (just dev).
• The test suite passes locally (just test).
• Pre-commit hooks are installed and pass (just precommit).
• I have added or updated tests where appropriate.
• I have updated the documentation / docstrings where appropriate.
• If my change touches the schema (tuttle/model.py), I generated and reviewed an Alembic migration (just migrate "<msg>"). N/A — no schema changes.
• If my change affects the graphical user interface, I have provided screenshots. N/A — documentation/checklist only.
• UI verification evidence is included for product UI changes, or this PR has no product UI surface.
• I understand and can explain all submitted changes; if AI assistance was significant, I have disclosed it in the summary above.

Command or review evidence:

python - <<'PY'
from pathlib import Path
root = Path('.')
agents = (root / 'AGENTS.md').read_text()
pr = (root / '.github/PULL_REQUEST_TEMPLATE.md').read_text()
required_agents = ['### UI verification playbook', 'When to run UI verification', 'Evidence to include', 'When not to over-test']
required_pr = ['UI verification evidence']
missing = [f'AGENTS.md: {s}' for s in required_agents if s not in agents]
missing += [f'PULL_REQUEST_TEMPLATE.md: {s}' for s in required_pr if s not in pr]
if missing:
    raise SystemExit('\n'.join(missing))
print('ok')
PY
python -m py_compile $(git ls-files '*.py')
git diff --check

I did not run just dev, just test, or just precommit because this only changes Markdown contributor guidance and the PR template; the focused Markdown presence check plus Python compile smoke and whitespace check cover the touched files without starting the app.