feat: allow configuring browser navigation wait state

[Disaster-Terminator]

Summary

• add a wait_until browser fetch option that maps to Playwright page.goto wait_until
• support the option for DynamicFetcher and StealthyFetcher, sync and async paths
• expose the option in CLI, shell signatures, docs, and agent skill references

Why

Some dynamic pages never reach Playwright default load state even though earlier navigation milestones are sufficient for extraction. Exposing wait_until lets callers choose commit, domcontentloaded, load, or networkidle without custom browser wrappers.

Tests

• uv run --with pytest --with pytest-httpbin pytest -q tests/fetchers/test_validator.py tests/cli/test_cli.py
• uv run --with pytest --with pytest-httpbin --with pytest-asyncio pytest -q tests/fetchers/async/test_dynamic.py tests/fetchers/sync/test_dynamic.py