feat(desktop): add duplicate session command

[anduimagui]

Issue for this PR

Closes #18118

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds the desktop-only /duplicate command, wires it into the session-scoped app providers so it can fork the current chat and carry the draft prompt into the new session, and documents the command in the app copy and commands docs. This is the desktop follow-up to the related TUI work in #12079.

How did you verify your code works?

Ran bun typecheck in packages/desktop and packages/app, and the original desktop commit also passed the repo typecheck hook during commit.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title desktop: add duplicate session command doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍