Make cbox entrypoint bootstrap timeout configurable

Problem

The cbox entrypoint bootstrap (uv sync during sandbox startup) uses hard-coded timeout values that cannot be adjusted without rebuilding the container image. In environments with slow networks or large dependency trees, the fixed timeout causes bootstrap to fail prematurely, leaving the sandbox in a broken state. Conversely, in fast environments the timeout may be unnecessarily generous, delaying failure detection. Operators have no way to tune this per-environment, which makes cbox unreliable across heterogeneous deployment targets.

Context

Possible Solutions

Plan

Implementation Progress

• Add env var(s) for bootstrap uv sync timeout with default preserving current behavior.
• Validate and sanitize timeout values in entrypoint shell logic.

• Document configuration in canonical docs.

• Operators can tune bootstrap timeout without rebuilding image.

• Invalid values degrade gracefully to defaults.

• cbox tests (and any added tests) pass.

• Source issue: hard-coded timeout may be too short/long for different environments.

Review Feedback

• [ ] Review cleared