Split ssh config into common plus a host layer

Putting the whole ssh config in common/ was wrong twice over. It mapped
git.rcjohnstone.com to localhost:2222, which is only true on mainframe
where Gitea runs -- on system76 it broke git entirely. And it replaced a
77-line file holding 13 personal host aliases (storagebox, several
forwarded localhost ports, older work hosts) with a five-entry fleet list.

common/ssh/config now Includes ~/.ssh/config.local first, so host entries
win on the first-value-obtained rule, and carries only the fleet aliases.
Each host keeps its own list; system76's is restored verbatim.

meta/hosts/mainframe.conf.yaml

@@ -10,6 +10,7 @@
       force: true
 
 - link:
+    ~/.ssh/config.local:    hosts/mainframe/ssh/config.local
     ~/.config/git/identity:     hosts/mainframe/git/identity
     ~/.config/paru:             common/paru
     ~/.config/rbw:              common/rbw

meta/hosts/nas.conf.yaml

@@ -10,5 +10,6 @@
       force: true
 
 - link:
+    ~/.ssh/config.local:    hosts/nas/ssh/config.local
     ~/.config/git/identity: hosts/nas/git/identity
     ~/.config/paru:         common/paru

meta/hosts/sctfw004.conf.yaml

@@ -10,6 +10,7 @@
       force: true
 
 - link:
+    ~/.ssh/config.local:    hosts/sctfw004/ssh/config.local
     ~/.config/wallust:       common/wallust
     ~/.config/hypr/colors.conf:  hosts/sctfw004/hypr/colors.conf
     ~/.config/waybar/colors.css: hosts/sctfw004/waybar/colors.css

meta/hosts/system76.conf.yaml

@@ -9,6 +9,7 @@
       force: true
 
 - link:
+    ~/.ssh/config.local:    hosts/system76/ssh/config.local
     ~/.config/wallust:       common/wallust
     ~/.config/hypr/colors.conf:  hosts/system76/hypr/colors.conf
     ~/.config/waybar/colors.css: hosts/system76/waybar/colors.css