fix: resolve import paths, remove async context managers, clean up CLI

- Change absolute imports to relative imports in gnmi module
- Remove async context manager methods (pygnmi is synchronous)
- Remove unimplemented --depth option from CLI
- Update documentation to reflect sync-only usage

src/cli.py

@@ -254,12 +254,6 @@ def discover_capabilities(
     default="all",
     help="Type of data to retrieve (default: all)",
 )
-@click.option(
-    "--depth", "-d",
-    type=int,
-    default=None,
-    help="Maximum depth to explore (not implemented yet)",
-)
 def discover_get(
     target: str,
     username: str,
@@ -268,7 +262,6 @@ def discover_get(
     output: str,
     path: str,
     data_type: str,
-    depth: int | None,
 ):
     """
     Get configuration or state data at a YANG path.