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>

src/fabledassistant/services/llm.py

@@ -337,7 +337,7 @@ async def build_context(
         tool_lines.append("When the user says 'remind me' with a time before an event, use the reminder_minutes parameter.")
         tool_lines.append("Use search_todos to find a specific CalDAV todo by keyword when list_todos would return too many results.")
     tool_lines.append("For relative dates like 'Friday' or 'next week', resolve them to YYYY-MM-DD format.")
-    tool_lines.append("When writing #tags in note bodies, use hyphens for multi-word tags (e.g. #science-fiction, #space-travel). Never use spaces inside a tag.")
+    tool_lines.append("When creating notes, use the `tags` parameter — do not embed #tag text in the note body.")
     tool_lines.append(
         "Use update_note to edit/expand an existing note OR to update a task's status/priority/due_date. "
         "Use create_note ONLY for genuinely new notes with a different title. "