ci: batch changed-file checks for Windows Git Bash

[schultzp2020]

Summary

• Batch changed-file format and lint checks within a Windows-safe command-line budget.
• Exclude .sh paths only from oxfmt targets; preserve package-name, test, and README gates.

Evidence

• Windows Git Bash, 111 changed paths: before → exit 1, The command line is too long.; after → exit 0 with four oxfmt batches.
• Regression covers spaces in paths, mixed and shell-only changes, every gate, and failure propagation.
• yarn format, yarn lint, yarn test, yarn check:changed, regression script, and pre-push hook passed.

Merge Danger

• CI-only scope: scripts/check-changed.sh and scripts/test-check-changed.sh; no changeset or version fields. No codemod release impact.
• Do not merge until exact-head GitHub Test and Changeset Check pass on ac50c6e2b85360978d38d161b4670a9cc6842e13.