feat(cli): add session worktree toggle

[bobleer]

Summary

• add /worktree, /worktree on|off|status|toggle, and the matching command palette action for the current chat session
• show the active Git branch and worktree state in the CLI header and startup guidance
• keep the project workspace separate from the active execution workspace so session restore, switching, and new-session creation preserve the correct scope
• use the existing core worktree lifecycle and surface retained-worktree safety results to the user

Compatibility

• the default Embedded TUI supports worktree lifecycle control
• Shared TUI reports worktree control as unavailable until its IPC contract exposes session workspace binding operations
• non-interactive exec behavior is unchanged

Testing

• cargo check -p bitfun-cli
• cargo test -p bitfun-cli (366 unit tests plus all CLI integration tests)