CBox sandbox bootstrap health parity for python and pre-commit

Problem

On fresh sandbox starts, the startup health checks for python and pre-commit report failures immediately after setup-worktree completes just install — even though the tools are actually installed and functional. The health probes look for executables in different paths than where the bootstrap actually places them (e.g., the probe checks system PATH while the tools are in .venv/bin/). This false-negative pattern causes healthy sandboxes to be marked as broken, triggering unnecessary recovery actions and eroding confidence in the health check system. The bootstrap setup and health probes need to agree on where to find interpreter and tool binaries.

Context

Possible Solutions

Plan

Implementation Progress

Implementation notes

• libs/cbox/entrypoint.sh now performs a minimal recovery path when full uv sync --extra dev fails:
• ensure .venv/bin/python exists via uv venv ...
• attempt uv pip install ... pre-commit
• This keeps bootstrap/runtime behavior aligned with startup health probes for python and pre-commit.
• Added deterministic tests in libs/cbox/test_entrypoint_bootstrap.py for:
• sync failure triggers minimal python/pre-commit bootstrap
• sync success skips minimal fallback path

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