Make the plain launcher install support quickstart

[abrichr]

Summary

• include Flow's browser contract in the base launcher dependency
• keep the existing openadapt[browser] route as a compatibility alias
• change the public quickstart to pip install openadapt
• make the clean lifecycle install the plain launcher wheel
• report a missing Playwright install as a broken or stale base install

Why

The bundled quickstart is the primary first-run path, but the base package did not install Playwright. A user who followed pip install openadapt reached a second-install failure. This change resolves that dependency at package installation time. It does not run pip from the CLI. Chromium remains lazy and downloads only on the first browser action.

The live website must keep the current extra command until the release that contains this change reaches PyPI. A follow-up web change can then switch the public command to the plain install.

Verification

• pytest -q: 182 passed, 6 skipped
• focused suite: 72 passed, 5 skipped
• Ruff check passes for every changed Python file
• lockfile validates with uv 0.11.29 and changes only the two base requirement records
• wheel and sdist pass scripts/verify_release_artifacts.py
• clean wheel lifecycle installs the wheel without an extra, resolves Flow 1.33.0 and Playwright 1.62.0, returns VERIFIED, confirms 2/2 effects, makes 0 model calls, emits the receipt, and verifies uninstall

Clean lifecycle summary: /tmp/openadapt-base-browser-lifecycle-20260826/summary.json