Remove legacy playground

[ZhengdaWang (zhengdawang443)]

Summary

• Remove the legacy Playground frontend, Python server, compatibility package, and tests.
• Make Control Plane through fsq ui the sole supported browser interface.
• Remove Playground from Vite, packaging, CI, launch configuration, documentation, and roadmap.
• Preserve existing Run data and historical design records.
• Add distribution and package-discovery assertions preventing accidental reintroduction.

Breaking Changes

The following surfaces are removed without compatibility shims:

• fsq_agent.playground
• fsq_agent.adapters.control_plane.playground
• Standalone Playground HTTP APIs
• Playground-only Goal, raw YAML, lifecycle editing, Run loading, and automatic Case publication workflows

Verification

• Frontend typecheck passed
• Frontend tests: 175 passed
• Production frontend build passed
• Ruff lint and format passed
• Python tests: 939 passed
• Distribution contract: 12 passed
• Wheel, sdist, and wheel-from-sdist builds passed
• Installed-wheel fsq ui HTTP smoke passed
• Independent SPEC implementation audit passed with no findings

The normal isolated uv build dependency fetch encountered a PyPI TLS handshake failure. The exact cached Hatchling 1.32.0 backend successfully completed all distribution builds.