feat: raw editor mode — plain textarea with frontmatter + wikilink autocomplete

Adds a third editor mode (alongside WYSIWYG and Diff) that shows the raw
markdown file in a monospaced textarea. Includes:

- useRawMode hook with derived state (no setState-in-effect) and onFlushPending
- RawEditorView: textarea with 500ms debounce, YAML error banner, wikilink
  autocomplete via [[  trigger, Cmd+S save
- BreadcrumbBar Code icon toggles raw mode; mutual exclusion with diff mode
- Command palette: "Toggle Raw Editor" (View group, requires active tab)
- useEditorModeExclusion hook extracted to keep Editor.tsx under complexity threshold
- buildViewCommands extracted from useCommandRegistry for same reason
- RawToggleButton and RawModeEditorSection components extracted for clean CC

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/utils/rawEditorUtils.ts

@@ -0,0 +1,21 @@
+/** Extract the wikilink query that the user is currently typing after [[ */
+export function extractWikilinkQuery(text: string, cursor: number): string | null {
+  const before = text.slice(0, cursor)
+  const triggerIdx = before.lastIndexOf('[[')
+  if (triggerIdx === -1) return null
+  const afterTrigger = before.slice(triggerIdx + 2)
+  // Don't trigger if the query contains ] (already closed) or a newline
+  if (afterTrigger.includes(']') || afterTrigger.includes('\n')) return null
+  return afterTrigger
+}
+
+/** Basic YAML frontmatter structural checks. */
+export function detectYamlError(content: string): string | null {
+  if (!content.startsWith('---')) return null
+  const rest = content.slice(3)
+  const closeIdx = rest.search(/\n---(\n|$)/)
+  if (closeIdx === -1) return 'Unclosed frontmatter block — add a closing --- line'
+  const block = rest.slice(0, closeIdx)
+  if (/^\t/m.test(block)) return 'YAML frontmatter contains tab indentation — use spaces'
+  return null
+}