clipboard: lift Cut/Copy/Paste from ted into the Editor (Command.Cut/Copy/Paste + key bindings)

[tig]

Today Cut, Copy, and Paste live in examples/ted/TedApp.EditCommands.cs. Per specs/clipboard/spec.md the Editor should own these as first-class commands so any consumer (clet, etc.) gets them for free.

Scope

• Add Command.Cut, Command.Copy, Command.Paste handlers in Editor.Commands.cs.
• Default keybindings on the Editor keymap: Ctrl+X / Ctrl+C / Ctrl+V (do not hard-code — register through Terminal.Gui's command key dictionary).
• Cut and Paste run inside a single Document.OpenUpdateScope () → one undo step.
• Multi-line paste preserves embedded line endings.
• ReadOnly blocks Cut/Paste; Copy still allowed.
• Resolve the [NEEDS CLARIFICATION] in FR-005: when there is no selection, default to no-op for Cut/Copy (matches VS Code's most-common preset). Document the decision in specs/decisions.md.
• Remove the now-redundant private Cut/Copy/Paste methods from TedApp.EditCommands.cs; the menu items already bind via Command.Cut/Copy/Paste, so they should pick up the Editor command automatically.

Definition of Done

• Round-trip tests: copy → paste across selection and caret cases.
• Multi-line paste test (LF + CRLF).
• Cut emits exactly one undo step (single Ctrl+Z restores text + selection).
• ReadOnly blocks Cut and Paste; Copy still works.
• Ted's Edit menu still works — the private wrappers are gone but the menu items invoke the Editor command.
• Decision logged in specs/decisions.md for the no-selection behavior.

Spec

specs/clipboard/spec.md