Gitignore SUMMARY.md signal file (#790)

PR #788's skill run committed SUMMARY.md to the branch alongside
its content commit (42267da) because /SUMMARY.md isn't in
.gitignore -- `git add -A` during claude-code-action's auto-commit
swept it up. The workflow's signals step reads and removes
SUMMARY.md, but only from the working tree; the commit that added
it sticks around.

/GAPS.md and /NO_CHANGES.md are already gitignored for exactly
this reason; SUMMARY.md was added in #785 but I missed adding the
matching gitignore entry.

With this in place, claude-code-action's `git add -A` will skip
SUMMARY.md, the skill still writes it to disk, the signals step
still reads + deletes it -- no commit involvement at all, which
is how GAPS.md and NO_CHANGES.md already work.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rdimitrov

.gitignore

@@ -23,9 +23,11 @@ yarn-error.log*
 .claude/settings.local.json
 
 # Signal files written by upstream-release-docs.yml for the skill to
-# communicate gaps or no-op releases back to the workflow. Never committed.
+# communicate gaps, no-op releases, or a change summary back to the
+# workflow. Never committed; the workflow reads and removes them.
 /GAPS.md
 /NO_CHANGES.md
+/SUMMARY.md
 .claude/scheduled_tasks.lock
 
 # Scratch workspace claude-code-action creates during skill runs.