Add /milestone command to cforge-dev chat

[ermin-muratovic]

Summary

Allow creating and managing milestones directly from cforge-dev chat
without leaving the planning session.

Commands to Add

/milestone create <title>     → creates milestone, prints URL
/milestone list               → lists all open milestones
/milestone assign <issue> <n> → assigns issue to milestone number

Why

Currently milestone creation requires dropping out to gh CLI.
The planning session should be self-contained — if you can plan
issues in chat, you should be able to organize them into milestones
without switching context.

Acceptance Criteria

• /milestone create works from inside cforge-dev chat
• /milestone list shows open milestones with issue counts
• /milestone assign works
• All operations use GitHubClient (no direct gh CLI calls)
• New session commands documented in /help output

Type

FEATURE