agent_hook_install_failed sent up to 200 chars of the raw installer error,
which embeds home-directory paths (and so the OS user name) and can echo
config contents. Send only the error's errno code or 'unknown', and enforce
that in the event schema. The anchored, capped errno pattern now lives in
src/shared/errno-code.ts and is shared with the renderer launch probe and the
file-write precondition instead of their own copies.
Fixes#21492
Co-authored-by: brynnclaw <261708852+brynnclaw@users.noreply.github.com>
Co-authored-by: Neil <neil@stably.ai>
