Add 'kugetsu server' command for managing git server configurations:
- kugetsu server list List all configured git servers
- kugetsu server add <name> <url> Add a new git server
- kugetsu server remove <name> Remove a git server
- kugetsu server default [<name>] Get or set default server
- kugetsu server get [<name>] Get URL for a server
Update get_repo_url() to use GIT_SERVERS config:
- First checks repos.json for direct mapping
- Then checks GIT_SERVERS for matching hostname
- Falls back to DEFAULT_GIT_SERVER
- Falls back to github.com as last resort
Update cmd_init to create config with default git servers:
- github.com -> https://github.com
- git.fbrns.co -> https://git.fbrns.coFixes#78
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