ci: post to mastodon (#201)

next attempt: filtering out the pr title string, hopefully corrected

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Chores**
  - Updated GitHub workflow script for Mastodon posting
  - Added checks for release pull requests
  - Improved version number extraction process
  - Enhanced error handling for missing PR title
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: cmeesters

.github/workflows/post_to_mastodon.sh

@@ -1,11 +1,24 @@
 #!/bin/bash
 
 # Extract version from PR tag passed as environment variable
-if [ -z "${PR_TITLE}" ]; then # apparently unset, workflow broken?
+if [ -z "${PR_TITLE}" ]; then
     echo "Error: 'PR_TITLE' environment variable is not set."
     exit 1
 fi
-version="${PR_TITLE##* }"
+
+# Check if this is a release PR
+if [[ ! "${PR_TITLE}" =~ ^chore\(main\):\ release ]]; then
+    echo "Not a release PR, skipping Mastodon post"
+    exit 0
+fi
+
+# Extract version (everything after "release ")
+if [[ "${PR_TITLE}" =~ [Rr]elease[[:space:]]+([0-9]+\.[0-9]+\.[0-9]+) ]]; then
+    version="${BASH_REMATCH[1]}"
+else
+    echo "Error: Could not extract version number from PR title"
+    exit 1
+fi
 
 # Validate version format
 if ! [[ $version =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]; then