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/forms.css

@@ -148,14 +148,16 @@
 
 .protocol-ssh .proto-only-ssh,
 .protocol-telnet .proto-only-telnet,
-.protocol-raw .proto-only-raw {
+.protocol-raw .proto-only-raw,
+.protocol-github .proto-only-github {
     display: flex;
 }
 
 /* For block-level protocol fields */
 .protocol-ssh .proto-only-ssh.form-section,
 .protocol-telnet .proto-only-telnet.form-section,
-.protocol-raw .proto-only-raw.form-section {
+.protocol-raw .proto-only-raw.form-section,
+.protocol-github .proto-only-github.form-section {
     display: block;
 }