docs: prep release v0.9.1 (#762)

Adds the dated release-notes file (.github/release-notes-20260530.md) for the
v0.9.1 patch cut. Single hotfix since v0.9.0: #732 CORS fix (drop
credentials:include from skill create/update/deprecate), so both packages go
0.9.0 → 0.9.1 in fixed mode.

Dated 20260530 deliberately: v0.9.0 shipped earlier the same UTC day with
.github/release-notes-20260529.md, and both release workflows select the
lexically-greatest 8-digit dated file (`sort -r | head -1`). A second
20260529 file is impossible (same name) and overwriting v0.9.0's historical
notes is wrong, so the next sortable date is used. The gate only validates the
most-recent file, not that its date equals today.

Ships an empty changeset (docs/CI-only) to satisfy check-changeset, same as
the v0.9.0 prep PR.

Closes #761.

Author: chronoai-shining

.changeset/release-notes-v0.9.1.md

@@ -0,0 +1,4 @@
+---
+---
+
+Release prep for v0.9.1 — adds the dated `.github/release-notes-20260530.md` used as the GitHub Release body. Docs/CI-only; no package version impact beyond the CORS hotfix changeset already on develop (#761).

.github/release-notes-20260530.md

@@ -0,0 +1,11 @@
+## Fixed
+
+- Saving, updating, or deprecating a skill no longer fails with a CORS error.
+
+## New Feature
+
+- Technical enhancement.
+
+## Changed
+
+- Technical enhancement.