Tree-Sitter Bash Security Analyzer Rewrite

The shell command security analyzer has been significantly rewritten to use tree-sitter AST analysis instead of relying solely on string-based heuristics. When tree-sitter is available, it provides authoritative quote context analysis, compound structure detection, and dangerous pattern identification.

New security checks include:

• Arithmetic expansion with variable references
• Heredocs with unquoted delimiters (shell expansion risk)
• Brace expansion syntax detection
• IFS assignment (word-splitting changes)
• Tilde expansion in assignment values
• Unicode whitespace and control character detection
• Zsh ~[ dynamic directory syntax
• Shell keyword misuse detection

When tree-sitter is unavailable, falls back to the legacy shell-quote parser with a log message: "tree-sitter unavailable, using legacy shell-quote path".