feat: add buffer-only squads release action

[dev-jodee]

Summary

• Add a prepare-squads-release composite action for buffer-only Squads releases.
• Transfer program and optional program-metadata buffer authority to the Squads vault without creating a proposal from CI.
• Document the new flow for teams that do not want to add a CI key as a Squads proposer.

Test Plan

• npm run lint
• ./node_modules/.bin/prettier readme.md prepare-squads-release/action.yaml --check
• ruby -e 'require "psych"; Psych.load_file("prepare-squads-release/action.yaml"); puts "ok"'