fix(ci): sort the import block in the migration ledger check

[DecisionNerd]

Python Quality has been failing on main since #1432 merged, blocking unrelated pull requests.

I001 [*] Import block is un-sorted or un-formatted
  --> scripts/ci/bazel-migration-ledger-check.py:9:1

#1432 added import collections immediately above import argparse, which ruff's I001 rejects. One line, moved one line down.

Two agents hit this while working other issues and both correctly diagnosed it as pre-existing and outside their change — git diff origin/main on that file was empty for them. It was pre-existing because I introduced it in #1432 and did not re-run the linter after editing the import block.

Verification

check	result
ruff check	All checks passed
ruff format --check	1 file already formatted
bazel-migration-ledger-check.py	OK: targets=129 exceptions=9
test-bazel-migration-ledger-check.py	passed

The gate's behaviour is unchanged — this is purely import ordering.

🤖 Generated with Claude Code

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