fix: tolerate sigpipe truncating large ai review diffs

[margaretjgu]

The diff-fetching step pipes to `head -c 32000` under `pipefail` (explicit in `ai-pr-review.yml`, implicit via GitHub Actions' default bash invocation in `ai-pr-review-external.yml`). Once `head` has its 32000 bytes it closes the pipe, the upstream `gh pr diff`/`gh api compare` gets SIGPIPE on its next write, and `pipefail` turns that into a step failure (exit 141), even though the truncated file is exactly what's wanted. Happens on any PR whose diff exceeds 32000 bytes, e.g. #537.

[github-actions]

The fix is correct. When head -c 32000 closes its stdin after reading 32000 bytes, gh gets SIGPIPE and exits non-zero, which was failing the pipeline. Adding || true suppresses that expected non-zero exit.

One thing to verify: the set -e / set -o pipefail behavior in GitHub Actions default shell (bash --noprofile --norc -eo pipefail). With pipefail, the pipeline exit code is the last non-zero from any command in the pipe. The || true applies to the whole pipeline, so it correctly swallows the SIGPIPE-caused failure from gh. This is the right fix.

Nothing else to flag here.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	2		0	0	0.23s
✅ COPYPASTE	jscpd	yes		no	no	0.39s
✅ REPOSITORY	gitleaks	yes		no	no	40.6s
✅ REPOSITORY	git_diff	yes		no	no	0.04s
✅ REPOSITORY	secretlint	yes		no	no	1.29s
✅ REPOSITORY	trivy	yes		no	no	18.97s
✅ YAML	yamllint	2		0	0	0.37s

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