Normalize linker path inputs to prevent absolute-path overrides

[Copilot]

linker.py previously hard-coded experiments and directly joined user-supplied directory values, which allowed values like /experiments or / to behave ambiguously or override the intended base path semantics. This change normalizes user path inputs to relative segments before joining under the proposal directory.

• Path normalization in linker flow

  • Added a small helper to coerce absolute-looking user input into a relative Path segment.
  • Replaced hard-coded "experiments" usage with normalized doc.get("experiments_directory", "experiments").
  • Applied the same normalization to experiment_alias_directory before join.

• Behavioral intent

  • Inputs such as "experiments", "/experiments", and "/" are treated as relative-to-proposal path components.
  • Joining no longer risks switching to an unintended absolute path root.

• Focused regression coverage

  • Added targeted unit tests for normalization and join behavior around relative, absolute, and root-like values.

def make_relative_path(path_value):
    path = Path(path_value)
    return Path(*path.parts[1:]) if path.is_absolute() else path

experiments_dir = make_relative_path(doc.get("experiments_directory", "experiments"))
path_expr = path_proposal / experiments_dir
path_expr_alias = path_expr / make_relative_path(path_expr_alias)