Use purr for recording release sound

[basnijholt]

Summary

• Change the macOS recording finish/release cue from Funk to Purr.
• Update the settings footer copy to describe the new start/end cue pairing.
• Add a focused cue-name assertion covering the Purr finish sound mapping.

Test Plan

• ruby -0777 -ne 'exit($_ =~ /case \\.finishedRecording:\\s*return NSSound\\.Name\\("Purr"\\)/m ? 0 : 1)' macos/AgentCLI/Sources/AgentCLI/RecordingSoundSettings.swift
• swift test
• git diff --check

Note: in this Command Line Tools environment, swift test completes a successful build but does not print XCTest execution output; xcodebuild test requires full Xcode.