refactor: Centralize version management in single source of truth

- Create src/skill_seekers/_version.py as single source of truth
- Read version dynamically from pyproject.toml at runtime
- Update all __init__.py files to import from _version module
- Add tomli dependency for Python <3.11 (built-in tomllib for 3.11+)
- Remove hardcoded version duplicates (2.7.2 in 3 files)
- Fixes version mismatch: pyproject.toml (2.7.4) vs __init__.py (2.7.2)

Benefits:
- Single place to update version (pyproject.toml)
- No more version mismatches across files
- Automatic version consistency
- Works across Python 3.10-3.13

Before:
- pyproject.toml: 2.7.4
- src/skill_seekers/__init__.py: 2.7.2
- src/skill_seekers/cli/__init__.py: 2.7.2
- src/skill_seekers/mcp/__init__.py: 2.7.2

After:
- pyproject.toml: 2.7.4 (single source of truth)
- All other files: import from _version.py

src/skill_seekers/cli/__init__.py

@@ -28,7 +28,8 @@ except ImportError:
     open_folder = None
     read_reference_files = None
 
-__version__ = "2.7.2"
+# Import centralized version
+from skill_seekers._version import __version__
 
 __all__ = [
     "LlmsTxtDetector",
@@ -36,4 +37,5 @@ __all__ = [
     "LlmsTxtParser",
     "open_folder",
     "read_reference_files",
+    "__version__",
 ]