fix(copilot): robust diff computation for cloud agent environments (#709)

The cloud agent checks out a branch like copilot/review-pr-NNN which
may not have upstream/main or origin/main refs available for merge-base.

Changes:
- Use gh pr diff as primary method (always works in PR context)
- Fall back to git merge-base for local non-PR usage
- Remove path filters (*.rs *.toml examples/) — review full diff
- Remove head -2000 truncation — let agents see everything
- Explicitly fetch origin/main in copilot-setup-steps.yml as backup

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/copilot-setup-steps.yml

@@ -8,3 +8,5 @@ steps:
   - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
     with:
       fetch-depth: 0 # full history needed for git diff against main
+  - run: git fetch origin main:refs/remotes/origin/main
+    name: Ensure origin/main ref is available for diff computation