docs(openspec): propose clipboard paste for plain text and EditorJS payload

[ilyamore88]

What

Planning only. This PR adds the clipboard-paste-plain-and-editorjs OpenSpec change, the first slice of #137. It contains no code changes. It is stacked on #203, the implementation of undo-group-by-task (proposed in #201). That change makes core record all changes of one browser task as one undo step.

The change touches sdk, ui and clipboard-plugin:

• New event. PasteUIEvent (ui:paste), which BlocksUI forwards from the native paste event.
• What paste reads. ClipboardPlugin prefers application/x-editor-js and falls back to text/plain.
  • Payload entries with unregistered tools are dropped, and block ids are discarded.
  • Multi-line text becomes one default-tool block per line, built with the default tool's importTextContent. Blank lines are skipped.
• Where content goes. Placement depends on the caret: start, middle (split) or end of a block, or replacing an empty default block. The whole paste is applied synchronously in the ui:paste handler, so core's per-task grouping records it as one undo step. The plugin uses no grouping API.
• Handing back to the browser. The plugin doesn't call preventDefault when it can't handle the paste: no clipboard data, nothing usable, or a selection across several inputs. The existing insertFromPaste path then runs unchanged.
• Copy (breaking change to the clipboard format).
  • Payload entries no longer include ids.
  • application/x-editor-js is written only for whole-block or multi-block selections. A selection inside one block copies as text/plain and text/html only.

Why

Paste currently goes through beforeinput → insertFromPaste as plain text only. Multi-line text ends up in a single block, and the EditorJS payload written on copy is never read. Copy also has two problems that would break paste once the payload is read:

• It writes block ids, and re-inserting them throws BlockAlreadyExistsError.
• It writes whole blocks even when the selection is inside a single input.

Notes for reviewers

• Deferred from Paste plugin #137: HTML and sanitizing, files, patterns, the tag registry, pasteConfig, read-only mode, deleting a selection across blocks, cut, drop, and clipping partially selected blocks when copying.
• Open question: should pasting over a whole-block selection replace that block? The design currently inserts after it.
• Collaboration. With collaboration-manager registered, a paste undoes in several steps. This comes from undo-group-by-task's non-goal.
• The change is not archived. Implementation happens via /opsx:apply clipboard-paste-plain-and-editorjs after undo-group-by-task.

[gohabereg]

I'm not clear why do we need to expose this to the public API

[github-actions]

Unit Tests

Package	Coverage	Delta
@editorjs/ot-server	N/A	N/A
@editorjs/editorjs	100%	0% ⚪️
@editorjs/core	73.87%	+0.73% 🟢
@editorjs/model	N/A	N/A
@editorjs/shortcuts-plugin	N/A	N/A
@editorjs/dom-adapters	N/A	N/A
@editorjs/collaboration-manager	N/A	N/A
@editorjs/clipboard-plugin	N/A	N/A
@editorjs/model-types	N/A	N/A

Mutation Tests

Package	Mutation score	Dashboard URL
@editorjs/core	86.00% 🟢	Dashboard

[github-actions]

Unit Tests

Package	Coverage	Delta
@editorjs/collaboration-manager	N/A	N/A

Mutation Tests

Package	Mutation score	Dashboard URL