feat: prevent environment variable leaks from sandbox execution

[harche]

Summary

• Layer 1: Replace process.env in the sandbox VM context with Object.freeze({}) instead of the real process.env. Modules loaded via safeRequire() still access the real env through their own module scope, so this doesn't break module functionality.
• Layer 2: Defense-in-depth output filter that scans each output line for sensitive env var values (≥8 chars, excluding known non-sensitive names/prefixes) before returning results. Blocks execution with a clear error if a leak is detected. Works across all execution modes: blocking, streaming, and test.
• 24 new tests covering both layers: empty env, frozen env, env enumeration prevention, output leak detection via console/stdout/stderr/errors, streaming mode, test mode, false positive avoidance, and more.

Test plan

• All 114 executor tests pass (including 24 new + all pre-existing)
• Existing module tests (kubernetes, prometheus, simple-statistics) still pass with restricted process.env
• Manual verification: submit code that tries to read process.env — should get undefined
• Manual verification: if a sensitive value somehow appears in output, execution is blocked with error

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