/build command to submit jobs

[abrichr]

Implement the /build command that lets users submit dev jobs via Telegram.

Acceptance criteria:

• /build <repo_url> [branch] submits a job to the Supabase queue
• Validates repo URL format
• Returns job ID and confirmation message
• Supports optional branch parameter (defaults to main)

[abrichr]

Implemented in PR #15 (merged to main as e7287b1).