Add session forking capability

[wpfleger96]

This PR adds session forking across CLI, backend API, and desktop UI. Users can now fork a session to create an independent copy and explore different conversation paths.

Forking lets you preserve conversation history while trying different approaches. For example, when debugging an issue, you can fork at a decision point to test different solutions without losing your original thread. Similar to how Claude Code handles session forking.

• Added POST /sessions/{id}/fork endpoint that uses SessionManager::fork_session() to copy sessions with the same name
• Implemented --fork CLI flag (requires --resume) with support for session ID, name, or path identifiers
• Fixed copy_session() to preserve provider_name and model_config when copying
• Created useForkSession hook that handles errors, loading states, and passes correct params to createChatWindow
• Added fork buttons in SessionListView and ChatInput that open forked sessions in new windows
• Added test_fork_session() to verify conversation and metadata preservation

Addresses #5865

[zanesq]

Just reiterating what @DOsinga said the desktop now has fork session support in chat

[wpfleger96]

@DOsinga @zanesq just simplified the PR based your feedback, what do y'all think now? I named this "fork" since that's what Claude Code calls the equivalent functionality I'm trying to implement, but I think I made it confusing since there's already a "Fork Session" in goose UI. what I want here is really "duplicate" instead of "fork"

truncate	copy	Result	Use Case
false	true	Full session copy	Duplicate for parallel work (my use case for this PR)
true	false	Edit in place	Truncate current session (existing Edit in Place UI option)
true	true	Branch at point	Copy + truncate at timestamp (exiting Fork Session UI option)