#42 (SLICE-1, reach): selection-aware Ask-Claude from editor body + tab

Document-edit flow SLICE-1 (specs/coauthoring-document-edit-flow.md §7.2,
INV-38): make "Ask Claude to Edit" reachable from the editor body AND the
editor tab, selection-aware — a selection routes to editSelection, no
selection to editDocument — both gated to markdown/authorable docs, both
flowing through the single runEditAndPropose path (no divergent edit code).

- package.json: add editSelection + editDocument to editor/context (selection-
  aware, markdown + file/untitled) and editor/title/context (selection-aware,
  resourceLangId == markdown). Markdown-gate the existing editor/context
  editSelection entry to match (its command handler is unchanged; the palette
  still reaches any authorable doc — see transcript Deferred decisions).
- trackChangesPreview.ts: cowriting.editDocument accepts the clicked tab's
  resource Uri (editor/title/context), targeting THAT document (opening it if
  needed) and falling back to the active editor when invoked with no arg —
  mirroring showTrackChangesPreview's #41 clicked-doc resolution.
- E2E (test/e2e/suite/f12Reach.test.ts): menu entries present, selection-aware,
  markdown-gated; editDocument(uri) targets the tab doc not the active editor;
  no-arg falls back to the active editor.
- docs/MANUAL-SMOKE-F12.md: SLICE-1 reach smoke steps.

208 unit + 65/5 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/trackChangesPreview.ts

@@ -82,10 +82,17 @@ export class TrackChangesPreviewController implements vscode.Disposable {
         }
         this.show(vscode.window.activeTextEditor?.document);
       }),
-      // F11: document-scoped Ask-Claude (also reused by #42's gateway). Edits the
-      // active markdown doc; the rewrite is diffed into per-hunk F4 proposals.
-      vscode.commands.registerCommand("cowriting.editDocument", () => {
-        const doc = vscode.window.activeTextEditor?.document;
+      // F11: document-scoped Ask-Claude (also reused by #42's reach gateways).
+      // Edits a markdown doc; the rewrite is diffed into F4 proposals.
+      // #42 (INV-38): the editor/title/context (tab) entry passes the clicked
+      // tab's resource Uri — target THAT document, opening it if it isn't already
+      // an open buffer (mirrors showTrackChangesPreview's #41 resolution); the
+      // palette / keybinding / editor/context pass nothing → the active editor.
+      vscode.commands.registerCommand("cowriting.editDocument", async (uri?: vscode.Uri) => {
+        const doc = uri
+          ? vscode.workspace.textDocuments.find((d) => d.uri.toString() === uri.toString()) ??
+            (await vscode.workspace.openTextDocument(uri))
+          : vscode.window.activeTextEditor?.document;
         if (!doc || !this.isMarkdown(doc)) {
           void vscode.window.showWarningMessage("Cowriting: open a Markdown document to ask Claude to edit it.");
           return;