Add GitHub polling for PR creation and assignee changes with separate Discord channels

[NickTheDevOpsGuy]

Description

Extend the Discord bot’s GitHub integration to support two independent polling streams:

2. New Pull Request creation notifications
3. Assignee change notifications for GitHub issues and PRs

Each stream should post messages to its own configurable Discord channel, allowing teams to separate signal (new work) from workflow changes (ownership updates).

Acceptance Criteria

• Notify a Discord channel when new PRs are opened
• Notify a separate Discord channel when assignees are added or removed
• Allow either stream to be enabled or disabled via environment configuration
• Avoid webhook requirements by using polling only (shared VPS–friendly)
• Prevent notification spam on bot restarts

Proposed Solution

No response

Screenshots / Mockups (optional)

No response

Additional Context

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Checklist

• I searched existing issues for duplicates
• This issue includes enough detail to act on