Extract the duplicated codeburn-binary lookup (PATH building, npx-shim skipping,
default lookup dirs, resolver) from menubar-installer and the Antigravity
statusline installer into src/persistent-codeburn.ts. Both now import it and
keep their own user-facing "install globally" message.
Match the Antigravity statusLine hook on the trailing agy-statusline-hook token
instead of a bare substring, so a custom command that merely mentions the token
is treated as custom (protected, backed up) rather than silently overwritten.
