has_hidden_execution: backtick check is not quote-aware (over-blocks quoted backticks)

[sanil-23]

Follow-up from @M3gA-Mind's review on PR tinyhumansai#2631.

Issue: has_hidden_execution blocks every backtick via command.contains(''), including inside a double-quoted literal (e.g. echo "use `code` syntax"), whereas contains_unquoted_background_ampersand(for&`) IS quote-aware. So a benign command with a quoted backtick is silently blocked by the in-tool guard after the human already approved it in the gate — the shell call just fails with no clear error.

File: src/openhuman/security/policy.rs::has_hidden_execution

Options:

• Make the backtick check quote-aware (mirror the & handling); or
• Keep the deliberate over-block (safe direction) — feat(agent): agentic coding runtime — gated OS capabilities (filesystem, shell, install) via deterministic permission tiers + chat approvals tinyhumansai/openhuman#2631 documents it as intentional. Filing for visibility/consistency.