fix(core): handle heredoc in command substitution guard

[liqiongyu]

Fixes #1698

What

• Teach detectCommandSubstitution() to understand heredocs.
• Allow substitution-like text in heredoc bodies when the delimiter is quoted (e.g. <<'EOF').
• Block $() / backticks in heredoc bodies when the delimiter is unquoted (because bash would execute them).

Why

Users saving sessions/logs to Markdown often include ``` and $()/backticks as plain text. The previous detector could false-positive on these, blocking safe file writes.

Test

• npm test -w packages/core -- src/utils/shell-utils.test.ts

[tanzhenxin]

@liqiongyu Thank you for this valuable PR! The heredoc support is a great addition to the shell command parser. We've reviewed the changes and identified a couple of issues that need to be addressed before we can merge:

---

Issue 1: Security vulnerability - Heredoc syntax in comments suppresses substitution detection [P1]

Problem:
The heredoc detection logic processes << operators whenever they appear outside quotes, without checking if the line is a comment. A commented heredoc like # <<'EOF' creates a pending quoted heredoc state. This causes subsequent lines to be treated as heredoc content (with substitution checks disabled), allowing malicious $(...) to bypass detection.

Attack Example:

# Fake heredoc <<'EOF' (this is just a comment)
$(rm -rf /)  # This executes but isn't flagged!
EOF

---

Issue 2: False positives for command substitution inside comments [P2 - Enhancement]

Problem:
The function reports command substitution patterns (e.g., $(date), `cmd`) even when they appear inside shell comments. Since bash does not execute substitutions in comments, these are false positives that incorrectly flag safe commands. This is a code quality improvement rather than a security fix.

Example:

# Note: $(rm -rf /) is dangerous
echo hello

This should be allowed, but currently may trigger detection.

---

Suggested Fix

Both issues can be addressed by adding inComment state tracking:

• Skip heredoc operator parsing inside comments (fixes Issue 1 - Security)
• Skip command substitution detection inside comments (fixes Issue 2 - False positives)

[liqiongyu]

@tanzhenxin
Thanks for the detailed review!

I addressed both items you pointed out:

1. Comments handling (P1 + P2):

• Added an inComment state so heredoc operator parsing and substitution detection are skipped inside shell comments.
• This prevents a commented fake heredoc like # <<'EOF' from creating a pending heredoc state, and avoids false positives for $(...) / backticks inside comments.
• Added regression tests for these cases.

2. Extra hardening:

• In unquoted heredoc bodies, bash ignores \\<newline> (line continuation), so $\\\\\n( becomes $( and can bypass a line-based scan.
• Added detection for this cross-line pattern (and tests), while still allowing the escaped form (\\$\\\\\n().

CI is green on all platforms now. Could you please take another look when you have time? Thanks!

[tanzhenxin]

LGTM!