fix: auto-detect codesign identity from keychain #15

Job	Run time
build (bun-darwin-x64, macos-latest, auggie-darwin-x64, auggie-darwin-x64)	1m 9s
build (bun-linux-x64, ubuntu-latest, auggie-linux-x64, auggie-linux-x64)	10s
build (bun-darwin-arm64, macos-latest, auggie-darwin-arm64, auggie-darwin-arm64)	49s
build (bun-windows-x64, ubuntu-latest, auggie-windows-x64.exe, auggie-windows-x64)	15s
release	8s
	2m 31s

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