Replace os.path with pathlib throughout codebase

Addresses review comment from gatecat: "while doing this refactor
probably worth also unifying the use of pathlib rather than os.path"

Changes:
- chipflow_lib/config/parser.py: Use Path(__file__).parent instead
  of os.path.dirname(__file__), remove os import
- chipflow_lib/platform/silicon.py: Use Path / operator for path
  joining instead of os.path.join(), convert to str where needed
  for compatibility with subprocess and string operations

All 5 instances of os.path usage in the codebase have been converted.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: robtaylor

chipflow_lib/config/parser.py

@@ -3,7 +3,6 @@
 Configuration file parsing and utilities.
 """
 
-import os
 import tomli
 
 from pathlib import Path
@@ -13,11 +12,11 @@
 from .models import Config
 
 def get_dir_models():
-    return os.path.dirname(__file__) + "/models"
+    return str(Path(__file__).parent / "models")
 
 
 def get_dir_software():
-    return os.path.dirname(__file__) + "/software"
+    return str(Path(__file__).parent / "software")
 
 
 def _parse_config_file(config_file) -> 'Config':

chipflow_lib/platform/silicon.py

@@ -8,6 +8,7 @@
 import subprocess
 import warnings
 
+from pathlib import Path
 from pprint import pformat
 from typing import TYPE_CHECKING, List, Generic, TypeVar
 
@@ -525,17 +526,17 @@ def build(self, elaboratable, name="top"):
             b"hierarchy"
         ]
 
-        build_dir = os.path.join(os.environ["CHIPFLOW_ROOT"], "build")
-        os.makedirs(build_dir, exist_ok=True)
+        build_dir = Path(os.environ["CHIPFLOW_ROOT"]) / "build"
+        build_dir.mkdir(parents=True, exist_ok=True)
 
         name = re.sub(r"[-_.]+", "_", name).lower()
-        link_script = os.path.join(build_dir, name + "_link.ys")
+        link_script = build_dir / (name + "_link.ys")
         with open(link_script, "wb") as script_fp:
             script_fp.write(b"\n".join(yosys_script))
-        output_rtlil = os.path.join(build_dir, name + ".il")
+        output_rtlil = build_dir / (name + ".il")
         subprocess.check_call([
             # yowasp supports forward slashes *only*
-            "yowasp-yosys", "-q", link_script.replace("\\", "/"),
-            "-o", output_rtlil.replace("\\", "/")
+            "yowasp-yosys", "-q", str(link_script).replace("\\", "/"),
+            "-o", str(output_rtlil).replace("\\", "/")
         ])
-        return output_rtlil
+        return str(output_rtlil)