chore: add release.sh script to automate version tagging

Tags the new version, updates all @vX.Y.Z references in README.md,
and prints a reminder to run the w3 release.sh.

Usage: ./release.sh [new-version]  (auto-increments patch if omitted)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: reuvenharrison

release.sh

@@ -0,0 +1,77 @@
+#!/bin/sh
+# release.sh — tag a new oasdiff-action version and update the README
+#
+# Usage: ./release.sh [new-version]
+#   e.g. ./release.sh v0.0.35
+#        ./release.sh        # auto-increments the patch version
+#
+# After running this, also run release.sh in the w3 repo to update the docs page.
+
+set -e
+
+REPO_DIR="$(cd "$(dirname "$0")" && pwd)"
+
+# ── Resolve version ──────────────────────────────────────────────────────────
+
+if [ -n "$1" ]; then
+  NEW="$1"
+else
+  LATEST=$(git -C "$REPO_DIR" tag --sort=-v:refname | grep '^v[0-9]' | head -1)
+  if [ -z "$LATEST" ]; then
+    echo "error: no existing tags found — provide a version explicitly" >&2
+    exit 1
+  fi
+  MAJOR=$(echo "$LATEST" | cut -d. -f1)
+  MINOR=$(echo "$LATEST" | cut -d. -f2)
+  PATCH=$(echo "$LATEST" | cut -d. -f3)
+  NEW="${MAJOR}.${MINOR}.$((PATCH + 1))"
+fi
+
+case "$NEW" in v*) ;; *) NEW="v${NEW}" ;; esac
+
+OLD=$(git -C "$REPO_DIR" tag --sort=-v:refname | grep '^v[0-9]' | head -1)
+
+if [ "$NEW" = "$OLD" ]; then
+  echo "error: new version ($NEW) is the same as the current tag" >&2
+  exit 1
+fi
+
+echo "Releasing $OLD → $NEW"
+
+# ── Validate git state ───────────────────────────────────────────────────────
+
+cd "$REPO_DIR"
+
+BRANCH=$(git branch --show-current)
+if [ "$BRANCH" != "main" ]; then
+  echo "error: not on main (currently on '$BRANCH') — check out main before releasing" >&2
+  exit 1
+fi
+
+git fetch origin main --quiet
+if [ "$(git rev-parse HEAD)" != "$(git rev-parse origin/main)" ]; then
+  echo "error: local main is not up to date with origin/main — run 'git pull' first" >&2
+  exit 1
+fi
+
+# ── Tag ──────────────────────────────────────────────────────────────────────
+
+git tag "$NEW"
+git push origin "$NEW"
+echo "✓ Tagged and pushed $NEW"
+
+# ── Update README.md ─────────────────────────────────────────────────────────
+
+sed -i '' "s|@${OLD}|@${NEW}|g" "$REPO_DIR/README.md"
+git add README.md
+git commit -m "chore: bump action version to ${NEW}"
+git push origin main
+echo "✓ Updated README.md and pushed"
+
+# ── Done ─────────────────────────────────────────────────────────────────────
+
+echo ""
+echo "Release $NEW complete."
+echo "  Tag:     https://github.com/oasdiff/oasdiff-action/releases/tag/${NEW}"
+echo ""
+echo "Next: run release.sh in the w3 repo to update the docs page (version: ${NEW})"