Move shared _INTERNAL_BASE/_internal_headers to src/tools/_common.py and
drop the duplicate _parse_tool_args (already in _common). tool_implementations.py
is now a pure re-export facade (+ 3 pre-existing email-context helpers, out of
scope). Domain files' function-local imports of these names still resolve via
the facade re-export.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Slice 1 Task 2 (#4082/#4071). Adds the package skeleton and moves the
shared _parse_tool_args helper into src/tools/_common.py. Domain modules
will import from here. tool_implementations.py is untouched at this step.