Strengthen README product hook

[afurm]

Summary

• Reworks the top of the README so first-time GitHub visitors see the product hook, one-command install, target audience, and core use cases immediately.
• Replaces the static preview placement with a fresh animated demo GIF generated from the local app flow.
• Adds a direct contributor path to open good first issue tickets.

Verification

• Ran Draftmora locally with a temporary demo SQLite database.
• Drove the board, task details, chat setup, and settings flow with Playwright.
• Generated .github/assets/draftmora-demo.gif from the recording.
• Ran git diff --check.