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/src/css/sidebar.css

@@ -185,6 +185,7 @@
 .sidebar-item .protocol-indicator.proto-ssh    { background: var(--protocol-ssh); }
 .sidebar-item .protocol-indicator.proto-telnet { background: var(--protocol-telnet); }
 .sidebar-item .protocol-indicator.proto-raw    { background: var(--protocol-raw); }
+.sidebar-item .protocol-indicator.proto-github { background: var(--protocol-github); }
 
 /* Text block (name + host) */
 .sidebar-item-text {