chore: sync external submodule pointers (#265)

* chore: sync external submodule pointers

Update external/hawk-mcpkit and external/hawk-core-contracts to
their latest main-branch commits, which now include:
- hawk-mcpkit: Validate() method for auth conflict detection
- hawk-core-contracts: review/session/scorecard improvements

* chore: update hawk-mcpkit submodule to include Validate() method

Point external/hawk-mcpkit at the commit that adds the
Validate() method for auth conflict detection. Update go.mod
and go.sum to match. hawk-core-contracts unchanged.

Author: Patel230