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.

hosts/system76/ssh/config.local

@@ -0,0 +1,75 @@
+# Personal host aliases for this machine. Kept verbatim from the
+# pre-refit ~/.ssh/config; common/ssh/config Includes this first.
+
+Host gitserver.idm.orionspace.com
+    PreferredAuthentications publickey
+    IdentityFile ~/.ssh/id_ed25519
+
+# Host gitlab.arcfield.com
+#     PreferredAuthentications publickey
+#     IdentityFile ~/.ssh/id_rsa
+
+Host github.com
+    HostName github.com
+    User git
+    PreferredAuthentications publickey
+    IdentityFile ~/.ssh/id_ed25519
+
+Host tiny
+    Hostname 192.168.1.188
+    User connor
+    IdentityFile ~/.ssh/id_ed25519
+
+Host devboard
+    HostName localhost
+    Port 9004
+
+Host rfbr
+    HostName localhost
+    User rfbr
+    Port 9003
+
+Host work
+    HostName localhost
+    Port 9000
+    User connor.johnstone@arcfield.com
+    IdentityFile ~/.ssh/id_ed25519
+
+Host traccs
+    HostName localhost
+    Port 9012
+    User azureuser
+    IdentityFile ~/.ssh/tracss-demo_key.pem
+
+Host orbitiq.space
+    Port 8001
+    User ubuntu
+    IdentityFile ~/.ssh/tracss.pem
+
+Host 18.188.195.42
+    Port 8001
+    User ubuntu
+    IdentityFile ~/.ssh/tracss.pem
+
+Host 3.31.241.208
+    Port 8001
+    User ubuntu
+    IdentityFile ~/.ssh/tracss.pem
+
+Host reverse.diff
+    HostName localhost
+    User ubuntu
+    Port 9005
+    IdentityFile ~/.ssh/ConnorKey.pem
+
+Host edu
+    HostName localhost
+    User rroci
+    Port 9007
+
+Host box
+    HostName u491382.your-storagebox.de
+    Port 23
+    User u491382
+    PreferredAuthentications publickey
+