GitHub activity feed — auto-post repo updates to a Discord channel

[BillChirico]

Part of #40

Monitor configured GitHub repos and post updates to a designated Discord channel.

Subtasks

• Config section: github.feed.{enabled, channelId, repos[], events[pr,issue,release,ci], pollIntervalMinutes}
• Polling engine: periodic gh CLI calls to check for new events since last poll timestamp
• PR embeds: title, author, status (open/merged/closed), files changed, review status — color-coded
• Issue embeds: title, author, labels, assignees — color-coded by label
• Release embeds: version, changelog preview, download links
• Dedup: track last-seen event IDs per repo to avoid duplicate posts across restarts
• Store poll state in DB (or fallback to file) for persistence across restarts
• /github add/remove/list — admin commands to manage tracked repos

Acceptance Criteria

• Repo activity posted to Discord
• No duplicate posts
• Configurable repos and event types