docs: document fix_args.py purpose and usage

[mayborn005]

Closes #1506

Summary

Documents contracts/fix_args.py so it is clear it is a one-time migration helper rather than part of the normal workflow.

Changes

• contracts/fix_args.py
  • Added a module docstring explaining what it patches (swapped commitment/uri args, mint-event encoding) and how/when to run it (python3 contracts/fix_args.py), and that re-running is a no-op.
• contracts/README.md
  • Added a "one-time migration helper" note describing invocation and that it is not part of the build/test workflow.
  • Also resolved a pre-existing merge-conflict marker in the Prerequisites section (kept the pinned-toolchain side).

Note

Out of scope per the issue: the script's logic was not modified.

[mayborn005]

✅ fix_args.py now has a docstring and a README note clarifying it is a one-time migration helper. @LabsCrypt this PR resolves issue #1506.

[mayborn005]

Hey @grantfox-oss — #1673 was rebased onto the latest main; the README now also cleans up conflict markers that were present in the base. Heads-up on the remaining red checks: contracts and Analyze (rust) fail on LabsCrypt:main itself (verified on both the PR base 4f5e77c and the latest main f5f3293), so they are pre-existing, not introduced by this PR. Ready for review.