[P2] Docs: fix install instructions & honest feature status

[pro-utkarshM]

Tasks

• README install: document fresh-venv + pip install -e ".[dev]" (post-Feat/the mvp #1).
• Mark self-healing as in-progress until update README.md #2 lands; don't overstate.
• Align README/proposal storage claims with Feature/healing loop #4 outcome.
• Note real language support status (Python stable; Java/JS beta until [P0] Implement bounded verify→repair self-healing loop #9).

Acceptance

Docs match shipped behavior.