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.

internal/models/defaults.go

@@ -11,6 +11,7 @@ const (
 	DefaultSSHPort    = 22
 	DefaultTelnetPort = 23
 	DefaultRAWPort    = 23
+	DefaultGitHubPort = 443
 )
 
 // Default timeout values (in seconds).
@@ -96,6 +97,14 @@ func NewProfile(protocol Protocol) Profile {
 			Encoding:       "utf-8",
 			PayloadPresets: []PayloadPreset{},
 		}
+	case ProtocolGitHub:
+		p.Port = DefaultGitHubPort
+		p.Host = "github.com"
+		p.GitHub = &GitHubConfig{
+			BaseURL:             "https://api.github.com",
+			SecretMode:          SecretModeSaved,
+			RefreshIntervalSecs: 300,
+		}
 	}
 
 	return p