feat(codebase-tools): add codebase exploration and file search tools

[jeancsil]

Introduce a CodebaseExplorer base class and three explorer tools
to let agents discover and inspect project source code.

• Add CodebaseExplorer with configurable root and ignore patterns to
  reduce noise (git, pycache, node_modules, venv, build, etc.).
• Add StructureExplorerTool exposing discover_structure to recursively
  list directories and files with depth control and file sizes.
• Add FileOutlinerTool exposing get_file_outline to extract class
  and function signatures from Python files for quick skimming.
• Add FileFragmentReaderTool exposing read_file_fragment to read a
  specific line range using "path:start:end" input format.
• Add initial code_searcher module scaffold for future search tools.

These changes provide foundational utilities for agentic tools to
navigate and inspect codebases programmatically, improving agent
context awareness and debugging capabilities.