fix(ci): unbreak spec-update workflow (EOF delimiter bug)

[EvanBacon]

Problem

The scheduled Update Apple Spec workflow has failed on every run for weeks (tracked in #30, ~60 failure comments). The workflow correctly detects new Apple spec versions (latest run found `4.3 → 4.3.1`) but crashes in the Read diff summary step:

##[error]Unable to process file command 'env' successfully.
##[error]Invalid value. Matching delimiter not found 'EOF'

Cause

The step writes a multiline heredoc to `$GITHUB_ENV`:

echo 'DIFF_SUMMARY<<EOF'
cat diff-summary.md
echo 'EOF'

`generateDiffSummary()` returns a string with no trailing newline, and `Bun.write` writes it verbatim. So `cat` emits content not ending in `\n`, and `echo 'EOF'` is glued onto the last line — GitHub never sees `EOF` standalone, the job fails, and the failure-notification job spams #30.

Net effect: no auto-update PR has been created since this broke, so the published spec is stuck at 4.3 while Apple is at 4.3.1.

Fix

• Emit an explicit blank line before the `EOF` delimiter in the workflow heredoc.
• Belt-and-suspenders: write `diff-summary.md` with a trailing newline.

Once merged, the next cron run should open the `spec/update-v4.3.1` PR and stop the failure comments.

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