feature-video: remove hardcoded R2 URL and require public URL verification

[zbeyens]

Problem

The feature-video command hardcodes a specific R2 public domain (pub-404...r2.dev) and assumes it works for all users.

In practice, users can upload successfully with rclone but still get broken demo links in PRs due to:

• different public R2 domains
• public URL disabled/misconfigured
• stale example domain in docs

What this changes

• Replaces hardcoded public URL assumptions with an explicit PUBLIC_BASE_URL variable.
• Adds a required pre-PR validation step:
  • build VIDEO_URL and PREVIEW_URL
  • curl -I both URLs
  • require HTTP 200 for both before updating the PR body.
• Replaces hardcoded URL in markdown example with <your-public-r2-domain> placeholder.
• Adds prerequisite note that a public R2 URL must be known.

Why this is objective and low-risk

• Docs-only change.
• No behavior changes outside the command guidance.
• Prevents false-success uploads that produce broken demo links.

Validation

• Verified markdown renders correctly.
• Verified command snippets are syntactically valid shell.

[kieranklaassen]

TY