Prepare v0.8.1 release

[mstallone]

TL;DR

Refresh the bundled pricing catalogs and record the approved changelog for the Runway 0.8.1 release.

What was happening

• The bundled LiteLLM and models.dev snapshots needed their release-time refresh.
• The Claude Sonnet 5 pricing spot-check still expected the model's previous catalog rates.
• The approved 0.8.1 release notes were not yet recorded in CHANGELOG.md.

What this changes

• Refreshes both bundled pricing snapshots from their live feeds.
• Updates the Claude Sonnet 5 pricing regression test to the current $2 input / $10 output rates and keeps alias-equivalence coverage.
• Prepends the approved v0.8.1 changelog with verified PR and author attribution.

Heads-up

• Version 0.8.1 has explicit owner approval.
• The release tag will be published only after this PR lands on main.

Tests

• swift build
• swift test — 1,441 tests passed, 4 skipped