Phase 20: Dedicated tag field — chip input, explicit tags array

Tags are now a first-class field rather than being auto-extracted from
the note body. A new TagInput.vue chip component handles tag entry in
both editor views with autocomplete, Enter/comma/backspace UX, and
space-to-hyphen sanitization.

Backend:
- routes/notes.py: create reads tags from JSON; update accepts explicit
  tags (omit = keep existing); append_tag writes to tags array with
  dedup; suggest-tags accepts current_tags filter; remove extract_tags
- routes/tasks.py: same — explicit tags on create/update; remove extract_tags
- services/tag_suggestions.py: current_tags param replaces body extraction
- services/tools.py: create_note tool schema adds tags param; executor passes it
- services/llm.py: system prompt tells LLM to use tags param, not embed #tag in body

Frontend:
- components/TagInput.vue: new chip-based tag input (autocomplete, keyboard UX)
- NoteEditorView.vue / TaskEditorView.vue: tags ref loaded from note.tags;
  TagInput placed between title and body; save/autosave include tags; suggest
  now adds chips; fetchTagSuggestions passes current_tags; dirty tracks tags
- TiptapEditor.vue: remove fetchTags prop and TagSuggestion extension;
  keep TagDecoration for legacy inline #tag highlighting

No DB migration needed — tags column already correct.

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

frontend/src/stores/notes.ts

@@ -77,6 +77,7 @@ export const useNotesStore = defineStore("notes", () => {
   async function createNote(data: {
     title: string;
     body: string;
+    tags?: string[];
   }): Promise<Note> {
     try {
       return await apiPost<Note>("/api/notes", data);
@@ -88,7 +89,7 @@ export const useNotesStore = defineStore("notes", () => {
 
   async function updateNote(
     id: number,
-    data: Partial<Pick<Note, "title" | "body">>
+    data: Partial<Pick<Note, "title" | "body" | "tags">>
   ): Promise<Note> {
     try {
       const note = await apiPut<Note>(`/api/notes/${id}`, data);