[1998x-stack]

Tool Factory path contract hardening.

• Add ToolDef.path_args for explicit, schema-backed path declarations (including custom names such as filename or asset_path).
• Canonicalize every declared or conventional filesystem path before executing a handler; fix old third-party fallback that only checked one field and discarded canonical paths.
• Preserve legacy built-in and conventional-name behavior; migrate built-in file, search and shell definitions to explicit declarations.
• Validate schema references in path_args at registration and re-validate approval-modified paths before execution.
• Regression tests cover differing process CWD, multi-path escape attempts, symlink escapes, custom field names and modified approval arguments.

Not an OS sandbox; symlink race/TOCTOU and arbitrary shell filesystem access remain out of scope.