Phase 1: Kerebron Markdown webview editor

[horner]

Summary

Implements Phase 1 of the plan: embedding the Kerebron editor as a Markdown
custom editor inside VS Code.

Changes

• Custom editor: MarkdownEditorProvider now serves a real webview instead of a placeholder <pre>. It:
  • Registers for .ededit files (Markdown content, so it no longer clashes with the built-in .md editor) at default priority.
  • Builds HTML with a locked-down CSP (nonce + wasm-unsafe-eval/unsafe-eval required by the tree-sitter emscripten glue).
  • Syncs the document ↔ ProseMirror via WorkspaceEdit, with a lastSyncedText echo guard to break the edit loop.
• Build: adds a Vite build:webview task (deno run -A npm:vite build) wired into build / vscode:prepublish; media/ build output is gitignored.
• Deno workspace: enumerates Kerebron package members, adds an import map (prosemirror, yjs, web-tree-sitter, vite, etc.), nodeModulesDir: auto, and raw-imports.
• Language: contributes the ededit language + language-configuration.json.
• Samples: adds samples/back-pain-encounter.md.

Plan status

Phase 1 checklist items are now checked off in plan.md.

Testing

• Manual: open a .ededit file, edit in the webview, confirm the document updates and external edits reflect back.