docs: add TUI fork command

[dannyward630]

Summary

• add /fork to the TUI command reference
• describe that it creates a new session from a selected point in the current conversation

Closes #21356

Verification

• bun run build from packages/web
• full pre-push bun turbo typecheck

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found a potential duplicate:

Related PR:

• docs(tui): document the /fork command #28628 - docs(tui): document the /fork command - docs(tui): document the /fork command #28628

This PR appears to be addressing the same documentation task—documenting the TUI /fork command. You should check if #28628 was already merged or closed, as current PR #32883 may be a duplicate effort.

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