fix(security): restore posixpath containment checks in path_mapper.py

Remove unused import os that triggers lint/typecheck failures and
replace os.path.relpath() usage with posixpath.relpath() throughout
path_mapper.py. Container paths are always POSIX — mixing os.path breaks
cross-platform correctness for non-POSIX hosts and was the sole functional
change on this PR branch (the actual security fix was already merged to master).

All reviewers flagged: unused import (ci/lint), wrong import domain
(ci/typecheck, ci/quality), and mismatch with master's posixpath-only
implementation.

Closes #7478
Refs: #11217

src/cleveragents/skills/inline_executor.py

@@ -263,7 +263,8 @@ class InlineToolExecutor:
                 try:
                     resolved = Path(value).resolve()
                     sandbox_resolved = sandbox_path.resolve()
-                    if not str(resolved).startswith(str(sandbox_resolved)):
+                    rel = os.path.relpath(str(resolved), str(sandbox_resolved))
+                    if rel.startswith(".."):
                         return (
                             f"Path '{value}' for key '{key}' escapes sandbox "
                             f"root '{sandbox_path}'"