Add launchers.<cmd>.sharedDirs: share a state-dir directory between launchers

Launchers with different ids share nothing, which is right for logins but
not for Claude's projects (sessions and memory) or Codex's memories. A
shared dir is a managed symlink to the other launcher's copy, created on
demand so the link holds whichever launcher starts first, and removed with
the declaration while the data stays.

Add packages: tools on every launcher's PATH

Add companions: every launcher of an environment callable from its sessions

Export AGENT_ENV_LAUNCHER, the launcher's command name

Add launchers with a state dir per login