Rename update feed infrastructure

[mstallone]

TL;DR

Rename the appcast/pricing backing branch and deployment workflow around their product purpose: the Runway update feed. Future releases also normalize the RSS channel title to Runway.

What was happening

• The gh-pages branch and Deploy Pages workflow were named after the hosting implementation, even though they carry both the Sparkle appcast and live pricing data.
• Sparkle's generated RSS channel title still identified the app as OpenUsage.
• Release and pricing documentation referred directly to the old backing branch.

What this changes

• Publish appcast and pricing updates to update-feed.
• Rename the deployment workflow to Deploy Update Feed while retaining GitHub's conventional github-pages environment.
• Normalize only the RSS channel title to Runway, preserving release item titles and historical enclosure URLs.
• Update repository docs, comments, and release/pricing skills for the new names.

Heads-up

• The public URLs remain unchanged.
• The existing gh-pages content will be copied to update-feed before merge; gh-pages will be removed only after the new deployment is live and verified.

Tests

• swift test — 1,442 XCTest cases passed, 4 skipped; Swift Testing suite passed.
• Focused appcast regression tests passed.
• Workflow YAML, shell syntax, JSON, and whitespace checks passed.