feat: GitHub Codespace integration with SSH tunnel support

[btessiau]

Summary

Add the ability to connect PolyPilot to a running GitHub Codespace, creating sessions that execute inside the codespace environment rather than on the local machine.

Motivation

Users working with GitHub Codespaces want to use PolyPilot's multi-session management while leveraging the codespace's environment (dependencies, tools, repo state). Currently the only option is Remote mode via DevTunnel, which requires manual setup inside the codespace.

Implementation (PR #308)

• Discovery: gh cs list --json enumerates available codespaces
• Connection: SSH tunnel (gh cs ssh) to copilot headless server running in the codespace
• Health monitoring: Background loop detects tunnel failures and auto-reconnects
• Session lifecycle: Sessions persist across app restarts; resume when tunnel reconnects
• Alpha gate: Disabled by default, enabled via Settings → Alpha → Codespaces toggle

Requirements

• GitHub CLI (gh) authenticated with codespace scope
• SSH server running in the codespace (typically via dotfiles configuration)
• Without SSH, the feature gracefully degrades to a "Setup Required" state — it cannot start copilot remotely

Known Limitations (Alpha)

1. SSH process orphaning on app crash — Spawned gh cs ssh processes survive hard crashes. No PID tracking for cleanup on next launch. Tunnels die when codespace times out.
2. Port-forward fallback is non-functional — gh cs ports forward cannot start copilot without SSH. Code correctly transitions to SetupRequired rather than hanging.
3. No integration tests for tunnel lifecycle — Core reconnect/health-check paths require real SSH/gh CLI. Model, contract, and isolation tests are comprehensive (113 tests).
4. No stale process cleanup on startup — Future improvement: track PIDs to clean up orphaned tunnels.

Files Changed

• CodespaceService.cs / .Lifecycle.cs / .Diagnostics.cs — SSH tunnel and codespace management
• CopilotService.Codespace.cs — Group lifecycle, health check, session resume
• CopilotService.cs / .Persistence.cs / .Bridge.cs / .Organization.cs — Integration points
• SessionSidebar.razor / CreateSessionForm.razor / SessionListItem.razor — UI
• Settings.razor — Alpha toggle
• ConnectionSettings.cs / SessionOrganization.cs — Model additions
• 8 test files (113 new tests)