fix(rag): close audit gaps in the in-house engine

An adversarial audit of the piragi -> in-house swap surfaced nine confirmed
issues; this fixes all of them.

- Re-ingest now REPLACES a source's chunks instead of appending. Add
  VectorStore.delete_by_source and BaseIndexPlugin.replace_on_reingest (default
  True), called before add_chunks in both ingest paths. Without it every
  startup / periodic / manual reindex appended a fresh copy of each doc's
  chunks, growing the tables unbounded and crowding out distinct results.
  Conversations opt OUT (replace_on_reingest=False): their many messages share
  one source URI, so delete-by-source would wipe history.
- index_* now honor the plugin IngestResult. The explicit record endpoints
  (error / standard / decision / review / learning) raise on failure instead of
  writing a green tracking row for content that never persisted;
  conversation / journal indexing stays best-effort but skips the tracking row
  when the embed fails. index_message / index_entry return IngestResult.
- A deprecated index type (code) now returns 404 instead of a 500 leaked from
  _get_plugin's missing-plugin error: add OptimalService.is_index_registered
  and guard the stats / clear / refresh routes. The panel drops the dead 'Code'
  category, filter, badge, label, and mock data.
- Panel: getContext reads 'results' (matches SearchResponse) instead of a
  non-existent 'context' field; the reindex toast no longer reports phantom
  '0 code files'; the stats 'Updated' label uses the max timestamp across
  indexes rather than indexes[0]; ProactiveContextItem matches the wire shape.
- Drop the always-zero per-document chunk_count from the documents API.
- Remove dead RAG settings (hybrid_search, cross_encoder) the engine never
  consumed, and correct stale piragi / BM25 references in code, README, and
  CLAUDE.md. Delete the unused duplicate roboco/kb embedder package the swap
  shipped.

Adds tests for replace-on-reingest (incl. the conversations carve-out) and the
deprecated-index 404.

tests/integration/test_optimal_routes.py

@@ -339,6 +339,7 @@ async def test_get_single_stats_invalid_type(optimal_client: AsyncClient) -> Non
 async def test_get_single_stats_success(optimal_client: AsyncClient) -> None:
     with patch("roboco.api.routes.optimal.get_optimal_service") as mock_get:
         mock_service = AsyncMock()
+        mock_service.is_index_registered = MagicMock(return_value=True)
         mock_service.get_index_stats = AsyncMock(
             return_value={
                 "index_type": "documentation",
@@ -354,6 +355,20 @@ async def test_get_single_stats_success(optimal_client: AsyncClient) -> None:
     assert response.status_code == HTTPStatus.OK
 
 
+@pytest.mark.asyncio
+async def test_get_single_stats_deprecated_index_returns_404(
+    optimal_client: AsyncClient,
+) -> None:
+    """A valid-but-unregistered index type (e.g. deprecated `code`) returns 404,
+    not a 500 leaked from _get_plugin's missing-plugin RuntimeError."""
+    with patch("roboco.api.routes.optimal.get_optimal_service") as mock_get:
+        mock_service = AsyncMock()
+        mock_service.is_index_registered = MagicMock(return_value=False)
+        mock_get.return_value = mock_service
+        response = await optimal_client.get("/api/optimal/stats/code", headers=_HDR)
+    assert response.status_code == HTTPStatus.NOT_FOUND
+
+
 @pytest.mark.asyncio
 async def test_check_staleness_via_http(optimal_client: AsyncClient) -> None:
     """`/stats/staleness` is now declared before `/stats/{index_type}`, so it
@@ -450,6 +465,7 @@ async def test_clear_index_invalid_type(optimal_client: AsyncClient) -> None:
 async def test_clear_index_success(optimal_client: AsyncClient) -> None:
     with patch("roboco.api.routes.optimal.get_optimal_service") as mock_get:
         mock_service = AsyncMock()
+        mock_service.is_index_registered = MagicMock(return_value=True)
         mock_service.clear_index = AsyncMock(return_value=None)
         mock_get.return_value = mock_service
         response = await optimal_client.delete(
@@ -523,6 +539,7 @@ async def test_refresh_index_invalid_type(optimal_client: AsyncClient) -> None:
 async def test_refresh_index_with_sources(optimal_client: AsyncClient) -> None:
     with patch("roboco.api.routes.optimal.get_optimal_service") as mock_get:
         mock_service = AsyncMock()
+        mock_service.is_index_registered = MagicMock(return_value=True)
         mock_service.refresh_index = AsyncMock(return_value=None)
         mock_get.return_value = mock_service
         response = await optimal_client.post(
@@ -538,6 +555,7 @@ async def test_refresh_index_empty_sources(optimal_client: AsyncClient) -> None:
     """Empty sources -> service.get_indexed_sources_for is called."""
     with patch("roboco.api.routes.optimal.get_optimal_service") as mock_get:
         mock_service = AsyncMock()
+        mock_service.is_index_registered = MagicMock(return_value=True)
         mock_service.get_indexed_sources_for = AsyncMock(return_value=["x.md"])
         mock_service.refresh_index = AsyncMock(return_value=None)
         mock_get.return_value = mock_service