fix(shell): block dangerous environment variables

[ambient-code]

Summary

• Adds a blocklist of security-sensitive environment variables (LD_PRELOAD, LD_LIBRARY_PATH, PATH, PYTHONPATH, BASH_ENV, KUBECONFIG, HOME) and dangerous prefixes (LD_*, BASH_FUNC_*) to the shell driver's _validate_script_params method
• Rejects any client-supplied env var matching the blocklist with a clear error message
• Adds 3 new unit tests covering blocked exact names, blocked prefixes, and safe variables still working

Fixes #359

Test plan

• All 17 shell driver tests pass (14 existing + 3 new)
• Linting passes with make lint-fix
• CI pipeline passes

🤖 Generated with Claude Code

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[ambient-code]

PR Review Status

CI Status: All checks are passing (build, pytest, e2e tests, lint, typos).

Review Comments: No unaddressed review comments found.

Code Review Notes:

• The blocklist approach for BLOCKED_ENV_VARS and BLOCKED_ENV_PREFIXES is clean and straightforward.
• The validation integrates correctly into the existing _validate_script_params method, running after the isidentifier() check.
• The LD_* prefix block covers the exact entries (LD_PRELOAD, LD_LIBRARY_PATH) as well as other dangerous LD variables like LD_AUDIT, LD_DEBUG, etc.
• The three new tests cover exact-match blocking, prefix-match blocking, and the positive case (safe vars still work).
• Error messages are clear and actionable.

No code changes needed at this time. The PR looks ready for maintainer review.

🤖 Generated with Claude Code