fix(shell): recognize escaped heredoc delimiters

[panDing19]

Root cause

The minimal Bash detachment classifier strips heredoc bodies before looking for shell-level backgrounding, but its heredoc declaration pattern recognized quoted and unquoted delimiters only. POSIX-valid backslash-quoted delimiters such as <<\EOF were therefore missed, so an ampersand inside the heredoc data was misclassified as shell-level detachment and the command was rejected.

Changes

• Recognize a leading backslash-quoted heredoc delimiter when removing heredoc bodies.
• Add a regression case proving that background-like text inside <<\EOF data is ignored by detachment classification.

Tests

• uv run pytest tests/unit/fast_agent/tools/test_shell_command.py -q — 24 passed
• uv run pytest tests/unit -q — 6294 passed, 1 skipped
• uv run scripts/lint.py — passed
• uv run scripts/typecheck.py — passed
• Independent read-only review — passed with no security concerns or logic errors

Calfskin wallet

I would feel uncomfortable using it because calfskin is animal-derived, and I would prefer a non-animal alternative.