fix: resolve AI chat empty body via || fallback + defensive body + Rust strip_frontmatter

Three root causes identified and fixed:

1. JS semantics bug: `activeNoteContent ?? allContent[path]` used nullish
   coalescing (`??`) which does NOT fall through on empty string ''. When
   handleEditorChange temporarily overwrites tab.content with frontmatter-
   only content during async content swaps, activeNoteContent becomes ''
   and the fallback to allContent never triggers. Fix: change `??` to `||`.

2. Defence-in-depth: when body is still empty after fallback (Tauri mode
   where allContent is {}), but wordCount > 0, the body field now includes
   an explicit get_note instruction instead of being empty. This is more
   reliable than the preamble instruction that Claude may skip.

3. Rust strip_frontmatter used `rest.find("---")` which matches `---`
   anywhere in text (including inside frontmatter values like `title:
   foo---bar`). Fixed to use `rest.find("\n---")` for line-boundary
   matching. This ensures accurate wordCount for the fallback heuristic.

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

src/utils/ai-context.test.ts

@@ -391,6 +391,37 @@ describe('buildContextSnapshot', () => {
     expect(json.activeNote.body).toBe('# Just a heading\n\nSome plain content.')
   })
 
+  it('falls back to allContent when activeNoteContent is empty string (|| fix)', () => {
+    // Regression: `??` does not fall through on empty string '', only on null/undefined.
+    // The `||` fix ensures empty string falls through to allContent.
+    const result = buildContextSnapshot({
+      activeEntry: active,
+      allContent,
+      entries,
+      activeNoteContent: '',
+    })
+    const json = JSON.parse(result.split('```json\n')[1].split('\n```')[0])
+    expect(json.activeNote.body).toContain('Project content.')
+    expect(json.activeNote.body).not.toBe('')
+  })
+
+  it('includes defensive body when body is empty but wordCount > 0', () => {
+    // When body is empty (e.g. timing issue) but note has content on disk,
+    // body field should instruct Claude to use get_note
+    const entryWithWords = makeEntry({
+      path: '/vault/a.md', title: 'Alpha', wordCount: 206,
+    })
+    const result = buildContextSnapshot({
+      activeEntry: entryWithWords,
+      allContent: {},
+      entries,
+      activeNoteContent: '---\ntitle: Alpha\n---\n',
+    })
+    const json = JSON.parse(result.split('```json\n')[1].split('\n```')[0])
+    expect(json.activeNote.body).toContain('get_note')
+    expect(json.activeNote.body).toContain('206 words')
+  })
+
   it('includes wikilink instruction in preamble', () => {
     const result = buildContextSnapshot({ activeEntry: active, allContent, entries })
     expect(result).toContain('[[Note Title]]')

src/utils/ai-context.ts

@@ -105,8 +105,18 @@ const MAX_NOTE_LIST_ITEMS = 100
 export function buildContextSnapshot(params: ContextSnapshotParams): string {
   const { activeEntry, allContent, activeNoteContent, openTabs, noteList, noteListFilter, entries, references } = params
 
-  const rawContent = activeNoteContent ?? allContent[activeEntry.path] ?? ''
-  const body = extractBody(rawContent)
+  // Use `||` (not `??`) so empty string '' falls through to allContent.
+  // This handles the case where handleEditorChange temporarily overwrites
+  // tab.content with frontmatter-only content during async content swaps.
+  const rawContent = activeNoteContent || allContent[activeEntry.path] || ''
+  let body = extractBody(rawContent)
+
+  // Defence-in-depth: when body is empty but the note has content on disk,
+  // include an explicit instruction in the body field itself (more reliable
+  // than a preamble instruction that Claude might skip).
+  if (!body && activeEntry.wordCount > 0) {
+    body = `[Content not available in editor context — use get_note("${activeEntry.path}") to read the full note (${activeEntry.wordCount} words)]`
+  }
 
   const snapshot: Record<string, unknown> = {
     activeNote: {