feat(gitea-axi): install the CLI and wire its agent context

Add gitea-axi as a flake input and a new modules.gitea-axi module that
installs the CLI through its home-manager module. That module also declares
the Claude Code context — the Agent Skill and the SessionStart dashboard hook
— when claude-code is enabled on the host, so neogaia gets both while a host
without the harness would get the CLI alone.

Drop the hand-written SessionStart hook from the claude-code module, which
named a gitea-axi binary nothing installed; the gitea-axi module owns it now.
Remove the vendored skill fork, which had drifted from upstream and would
collide with the copy the module writes.

modules/claude-code/claude-code.nix

@@ -77,18 +77,6 @@ in
                 ];
               }
             ];
-            SessionStart = [
-              {
-                matcher = "";
-                hooks = [
-                  {
-                    type = "command";
-                    command = "gitea-axi";
-                    timeout = 10;
-                  }
-                ];
-              }
-            ];
           };
         };
       };