fix(bash-guard): deny git verbs hidden in command substitutions

[rennf93]

Closes the command-substitution bypass that antfleet-ops flagged in #223 — credit to them for the finding. A denied git verb inside $(...) or backticks is expanded by the shell before the wrapping echo/printf runs, so the skeletonizer's strip hid it from the git check.

This supersedes #223, whose submitted fix could not land as-is:

• it targeted the correct source file, but ran the new deny unconditionally — on Grok agents (hook runs with ROBOCO_GUARD_SKIP_GIT=1 for graceful --deny) it would exit 2 and hard-cancel the run;
• it scanned the raw command, so a single-quoted / heredoc literal $(git …) was a false positive;
• it failed open on a parse error.

This rework:

• runs inside the ROBOCO_GUARD_SKIP_GIT guard, so on Grok it stays the native --deny's job (never a run-cancelling deny);
• excludes single-quoted strings and heredoc bodies (literal / data, matching the skeletonizer), so a README documenting git verbs isn't a false positive;
• fails closed (a non-sentinel / Python failure denies).

44 bash-guard tests pass (5 new: dollar / backtick / double-quoted substitution deny, single-quoted literal allow, Grok-skip allow).

Once this merges, #223 should be closed with a link here.

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