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.

common/ssh/config

@@ -1,11 +1,8 @@
-# Host aliases only. No keys, no secrets.
-
-Host git.rcjohnstone.com
-    HostName localhost
-    Port 2222
-    User connor
-    IdentityFile ~/.ssh/id_ed25519
+# Host-specific aliases come first: ssh takes the FIRST value it obtains for
+# each keyword, so anything in config.local wins over the fleet defaults below.
+Include ~/.ssh/config.local
 
+# --- the fleet -------------------------------------------------------------
 Host nas
     HostName 10.0.0.3
     User connor
@@ -13,6 +10,8 @@ Host nas
 Host mainframe
     HostName 10.0.0.2
     User connor
+    ServerAliveInterval 30
+    ServerAliveCountMax 6
 
 Host system76
     HostName 10.0.0.4
@@ -21,3 +20,11 @@ Host system76
 Host work_laptop
     HostName 10.0.0.8
     User connorjohnstone
+
+# Correct from every machine except mainframe itself, which overrides it in
+# hosts/mainframe/ssh/config.local — there, Gitea is a local port.
+Host git.rcjohnstone.com
+    HostName 10.0.0.2
+    Port 2222
+    User connor
+    IdentityFile ~/.ssh/id_ed25519