feat(rag): RAG scoping and context isolation controls

- Migration 0030: add conversations.rag_project_id (NULL=orphan-only,
  -1=all notes, positive=project), projects.auto_summary and
  projects.summary_updated_at
- Three-value scope semantics thread from build_context() → semantic
  search and keyword fallback via orphan_only + effective_project_id
- Project summarization background job (generate_project_summary,
  backfill_project_summaries) called via Ollama; triggered on project
  update and note saves (debounced 1h); runs at startup
- New LLM tools: search_projects (SequenceMatcher scoring on
  title+description+auto_summary) and set_rag_scope (persists to DB,
  workspace-guarded, emits new_rag_scope in SSE done event)
- execute_tool() accepts conv_id + workspace_project_id; generation_task
  passes both and captures scope changes for SSE done enrichment
- Frontend: Conversation type gets rag_project_id; chat store adds
  ragProjectId computed + updateRagScope(); SSE done handler syncs scope
- ChatView: replace sidebar ProjectSelector with a scope chip pill above
  the input bar, animated dropdown, pulse on model-driven scope change

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

src/fabledassistant/services/embeddings.py

@@ -81,6 +81,7 @@ async def semantic_search_notes(
     threshold: float = _SIMILARITY_THRESHOLD,
     project_id: int | None = None,
     is_task: bool | None = None,
+    orphan_only: bool = False,
 ) -> list[tuple[float, Note]]:
     """Return up to *limit* (score, note) pairs most relevant to *query*.
 
@@ -101,7 +102,9 @@ async def semantic_search_notes(
                 .join(Note, NoteEmbedding.note_id == Note.id)
                 .where(NoteEmbedding.user_id == user_id)
             )
-            if project_id is not None:
+            if orphan_only:
+                stmt = stmt.where(Note.project_id.is_(None))
+            elif project_id is not None:
                 stmt = stmt.where(Note.project_id == project_id)
             if is_task is True:
                 stmt = stmt.where(Note.status.isnot(None))