feat: zsh as default shell, pkgar staging fix

Shell: Changed default shell from ion to zsh for all user accounts
in base.toml, redbear-mini.toml, redbear-full.toml, and greeter config.
zsh 5.9 is already ported and builds (ZSH-PORTING-PLAN — fully implemented).
ion is kept as fallback/alternative.

Cookbook: pkgar staging fallback — when a dependency's target pkgar
doesn't exist, fall back to repo/<target>/<pkg>.pkgar. This fixes the
kf6-kitemviews build failure where libwayland's pkgar was missing from
the target directory.

config/redbear-mini.toml

@@ -19,7 +19,7 @@ uid = 100
 gid = 100
 name = "messagebus"
 home = "/nonexistent"
-shell = "/usr/bin/ion"
+shell = "/usr/bin/zsh"
 
 [packages]
 # Red Bear OS branding and host utilities.