feat: add Discord notification workflow for PR and Issue events

[masami-agent]

Summary

Adds a GitHub Actions workflow that sends notifications to Discord when PRs or Issues are opened.

This is the implementation counterpart to the docs in PR #105.

What it does

• Triggers on pull_request (opened/reopened) and issues (opened)
• Sends a structured [GH-EVENT] message to Discord via $DISCORD_WEBHOOK_URL secret
• Message format follows the convention defined in docs/github-webhook-integration.md

Requirements

• DISCORD_WEBHOOK_URL must be set as a repository secret

Example output

[GH-EVENT] repo:openabdev/openab action:pr_opened PR #105
**docs: add GitHub webhook integration and token setup guides**
by masami-agent
https://github.com/openabdev/openab/pull/105