test: add webapp first-run e2e coverage

[c4pt0r]

Summary

• add opt-in docs/webapp E2E pytest coverage for the first-run guide and integrated webapp startup
• register an e2e pytest marker
• update integrated webapp missing-dist guidance and webapp/setup.sh from npm install to npm ci
• document that lane E2E tests may intentionally overlap the core harness when they exercise the same user-visible behavior

E2E coverage

Task #27 covers the docs/webapp E2E lane:

• first-run guide command parity
• source server make dev config fallback command
• uv sync --extra webapp
• integrated webapp missing-dist fail-fast path
• real npm ci && npm run build
• real uv run agfs-shell --webapp subprocess with HTTP fetch of /

Review

Cross-review PASS from @dev-2 in Slock task #27. I folded in his non-blocking docs/e2e clarification about lane tests overlapping the core harness.

Verification

• git diff --check HEAD~1..HEAD
• PYTHONPATH=. .venv/bin/python -m pytest tests/test_webapp_server.py tests/test_docs_webapp_e2e.py -v --timeout=15 -> 2 passed, 1 skipped
• AGFS_RUN_E2E=1 PYTHONPATH=. .venv/bin/python -m pytest tests/test_docs_webapp_e2e.py -v --timeout=180 -> 1 passed

Part of stability issue #21. Closes task #27.

[c4pt0r]

Closing this PR in favor of the task #27 v2 patch, which integrates the docs/webapp E2E lane into the shared task #24 harness.