Add GitHub connection type — repo viewer with org grouping and auto-refresh

New protocol type 'github' for browsing repositories via the GitHub REST API.
Creates a non-terminal panel showing repos grouped by owner/org, sorted by
last commit date (newest first), with search filtering and auto-refresh.

Backend:
- ProtocolGitHub constant and GitHubConfig/GitHubRepo model structs
- New internal/githubclient package (REST API, pagination, token validation)
- Session manager connectGitHub() with periodic refresh goroutine
- API surface: RefreshGitHubRepos() for on-demand refresh
- Token stored via existing credential infrastructure (keychain/vault)

Frontend:
- GitHub tab in profile editor with API URL and token storage fields
- New github.js viewer component (grouped repos, badges, language dots)
- github.css with charcoal-themed repo cards and org headers
- Protocol colour tokens, tab badge, sidebar indicator for GitHub
- Event wiring for session:github:repos and session:github:error
- Cleanup on tab close

Supports GitHub Enterprise via configurable API base URL.

frontend/index.html

@@ -18,6 +18,7 @@
     <link rel="stylesheet" href="/src/css/terminal.css">
     <link rel="stylesheet" href="/src/css/forms.css">
     <link rel="stylesheet" href="/src/css/modals.css">
+    <link rel="stylesheet" href="/src/css/github.css">
 </head>
 <body>
     <div id="app" class="app">
@@ -170,6 +171,7 @@
     <script src="/src/js/terminal.js"></script>
     <script src="/src/js/profiles.js"></script>
     <script src="/src/js/raw.js"></script>
+    <script src="/src/js/github.js"></script>
     <script src="/src/js/settings.js"></script>
     <script src="/src/js/commands.js"></script>
     <script src="/src/js/secrets.js"></script>