chore: remove playwright and sol screenshot CLI
Visual review of the Convey UI now happens via external pinchtab; the in-repo sol screenshot tool and its Playwright dependency are dead weight.
- Delete convey/screenshot.py and apps/chat/tests/test_markdown_bootstrap_browser.py
- Drop playwright from pyproject.toml; uv.lock loses playwright, pyee, greenlet
- Remove the playwright install chromium step from make install
- Trim screenshot from think/sol_cli.py COMMANDS and the Convey help group
- Trim doc references in AGENTS.md, README.md, docs/testing.md, docs/CONVEY.md, docs/SOLCLI.md, docs/BACKLOG.md, and scripts/doctor.py
- Trim the stale make screenshot clause from tests/integration/test_apps.py