refactor: split constants.py (2876 lines) into submodules

[vitali87]

Problem

codebase_rag/constants.py is 2876 lines, far exceeding the project's 700-line soft limit. It contains unrelated concerns: file extensions, tree-sitter AST node types, CLI messages, language metadata, C++ operator maps, stdlib type maps, color enums, and more.

Suggested Structure

Split into a constants/ submodule:

• constants/__init__.py - re-exports for backwards compatibility
• constants/ast_nodes.py - tree-sitter node type constants (TS_IDENTIFIER, TS_ATTRIBUTE, etc.)
• constants/languages.py - SupportedLanguage, language metadata, file extensions
• constants/operators.py - C++ operator maps, stdlib type maps
• constants/graph.py - NodeLabel, RelationshipType, key names
• constants/cli.py - Color, StyleModifier, CLI messages, UI formatting

Affected Files

• codebase_rag/constants.py (source of split)
• All files importing from codebase_rag.constants or using the cs alias (widespread)