Improve multiline shell-command analysis

[Pavithran-R-A]

Problem

SetupSentry does not fully parse multiline shell constructs. Some multi-line dangerous patterns may not be detected, including:

• Heredocs with embedded commands
• Line continuations that split a dangerous command across lines
• Complex variable expansion that hides command execution
• Pipelines that span multiple lines

Proposed Scope

Improve the markdown code block scanner to handle more multiline shell patterns:

• Detect dangerous commands split across lines with backslash continuation
• Detect heredoc patterns that pipe to shell
• Improve pipeline detection across line boundaries

Files/Components

• src/markdown.ts — code block extraction and line scanning
• src/rules/ — rules that may need multiline context
• tests/rules.test.ts — new multiline test cases

Acceptance Criteria

• At least 3 new multiline pattern test cases added
• Scanner detects dangerous commands split across 2+ lines
• No increase in false positives on existing safe content
• All existing tests still pass
• README limitations section updated