Fix local mode handling of empty loader environment variables

[sylvesterkaczmarek]

Summary

Tier 3 local mode rejects process-loader environment variables before considering their values. The evaluator itself supplies empty loader variables such as BASH_ENV, so a local run can fail before the agent process starts.

This drops blocked loader variables when their value is empty while preserving the existing rejection for every non-empty value.

Closes #136.

Verification

• I am familiar with the Contributing Guidelines
• Added or updated focused tests
• Updated documentation for user-visible changes
• Ran make lint
• Ran make test
• Ran make build
• Did not add credentials, private datasets, or proprietary benchmark content

Focused local-mode tests: 16 passed. Ruff and package build pass.

make test completed with 6034 passed, 17 skipped, and 5 failures unrelated to this change. The CLI golden failure and the three Anthropic httpx/httpx2 failures reproduce on unmodified current main; the remaining DNS deadline test is timing-sensitive and passes when rerun alone.

Release Impact

• No user-visible release note needed
• Updated CHANGELOG.md

[rng1995]

Thanks for the fix. This behavior is now covered on main by #133, including the empty process-loader resets and focused regression tests, so this PR has been superseded and can be closed.

[sylvesterkaczmarek]

Thanks. Closing as superseded by #133.