#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>

package.json

@@ -120,10 +120,20 @@
         }
       ],
       "editor/title/context": [
+        {
+          "command": "cowriting.editSelection",
+          "when": "editorHasSelection && resourceLangId == markdown",
+          "group": "1_cowriting@1"
+        },
+        {
+          "command": "cowriting.editDocument",
+          "when": "!editorHasSelection && resourceLangId == markdown",
+          "group": "1_cowriting@1"
+        },
         {
           "command": "cowriting.showTrackChangesPreview",
           "when": "resourceLangId == markdown",
-          "group": "1_cowriting"
+          "group": "1_cowriting@3"
         }
       ],
       "explorer/context": [
@@ -136,7 +146,12 @@
       "editor/context": [
         {
           "command": "cowriting.editSelection",
-          "when": "editorHasSelection && (resourceScheme == file || resourceScheme == untitled)",
+          "when": "editorHasSelection && editorLangId == markdown && (resourceScheme == file || resourceScheme == untitled)",
+          "group": "1_cowriting@1"
+        },
+        {
+          "command": "cowriting.editDocument",
+          "when": "!editorHasSelection && editorLangId == markdown && (resourceScheme == file || resourceScheme == untitled)",
           "group": "1_cowriting@1"
         },
         {