Feature Request: Add session fork functionality

[wpfleger96]

Motivation

After finishing work in a session, I often want to branch into parallel tasks that share the same context. For example, after implementing a feature, I want to simultaneously write tests (in one session) and docs (in another) - both need the implementation context but should run independently.

Currently this requires re-explaining context in new sessions or working sequentially.

Proposed Solution

Add a --fork flag:

goose session --resume --fork
goose session --resume --name <session-name> --fork
goose session --resume --session-id <session-id> --fork

Creates a new session with all messages copied from the source, allowing independent continuation.

Alternatives Considered

• Manual DB duplication (error-prone, not user-friendly)
• Export/import session JSON (more complex than needed)

Additional Context

• Claude Code has this via --fork-session - proven UX

• Desktop UI: fork button in session history would be a nice future addition

• [x] I have verified this does not duplicate an existing feature request

[DOsinga]

we have the functionality in the desktop (/cc @zanesq) - there's now a primitive to copy a conversation (which it looks like what you want) and a truncate (in the desktop you can edit a message further back in time and then fork from there, or wipe the rest of the conversation and continue with the new message).

[wpfleger96]

@DOsinga / @zanesq I took a stab at implementing this in #5882 which builds on top of that copy session primitive - wondering what you think!