Fix YAML indentation bug in AI prompt template substitution

actions/ai-inference@v1 substitutes {{var}} placeholders as raw text
into the prompt YAML file before parsing it as YAML. Since the diff/
diff-stat files started at column 0, injecting their multi-line
content broke the indentation of the enclosing `content: |-` block
scalar in changelog-entries.prompt.yml (which expects continuation
lines to be indented), causing "Failed to parse prompt file: bad
indentation of a mapping entry" at runtime, e.g.:

https://github.com/slack-ruby/slack-ruby-client/actions/runs/31552877245/job/93979138338

Indent every line of the diff/diff-stat files with 6 spaces (to
match the prompt file's block scalar indentation) before writing
them out, so the substituted content stays inside the block.

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

Author: dblock

.github/workflows/update_api.yml

@@ -44,8 +44,8 @@ jobs:
       - name: Prepare diff for AI summary
         if: steps.changes.outputs.changed == 'true'
         run: |
-          git diff --stat > /tmp/diff_stat.txt
-          git diff | head -c 20000 > /tmp/diff.txt
+          git diff --stat | sed 's/^/      /' > /tmp/diff_stat.txt
+          git diff | head -c 20000 | sed 's/^/      /' > /tmp/diff.txt
       - name: Generate changelog entries with AI
         if: steps.changes.outputs.changed == 'true'
         id: ai