fix: pass project_hint into retrieve and add path-signal ranking

Two changes that belong together:

1. builder.build_context() now passes project_hint into retrieve(),
   so the project-aware boost actually fires for the retrieval pipeline
   driven by /context/build. Before this, only direct /query callers
   benefited from the registered-project boost.

2. retriever now applies two more ranking signals on every chunk:
   - _query_match_boost: boosts chunks whose source/title/heading
     echo high-signal query tokens (stop list filters out generic
     words like "the", "project", "system")
   - _path_signal_boost: down-weights archival noise (_archive,
     _history, pre-cleanup, reviews) by 0.72 and up-weights current
     high-signal docs (status, decision, requirements, charter,
     system-map, error-budget, ...) by 1.18

Tests:
- test_context_builder_passes_project_hint_to_retrieval verifies
  the wiring fix
- test_retrieve_downranks_archive_noise_and_prefers_high_signal_paths
  verifies the new ranking helpers prefer current docs over archive

This addresses the cross-project competition and archive bleed
called out in current-state.md after the Wave 1 ingestion.

tests/test_retrieval.py

@@ -118,3 +118,58 @@ def test_retrieve_project_hint_boosts_matching_chunks(monkeypatch):
     assert len(results) == 2
     assert results[0].chunk_id == "chunk-a"
     assert results[0].score > results[1].score
+
+
+def test_retrieve_downranks_archive_noise_and_prefers_high_signal_paths(monkeypatch):
+    class FakeStore:
+        def query(self, query_embedding, top_k=10, where=None):
+            return {
+                "ids": [["chunk-archive", "chunk-requirements"]],
+                "documents": [["archive doc", "requirements doc"]],
+                "metadatas": [[
+                    {
+                        "heading_path": "History",
+                        "source_file": "p05-interferometer/pkm/_archive/old/Error-Budget.md",
+                        "tags": '["p05-interferometer"]',
+                        "title": "Old Error Budget",
+                        "document_id": "doc-a",
+                    },
+                    {
+                        "heading_path": "Overview",
+                        "source_file": "p05-interferometer/pkm/Requirements/Error-Budget.md",
+                        "tags": '["p05-interferometer"]',
+                        "title": "Error Budget",
+                        "document_id": "doc-b",
+                    },
+                ]],
+                "distances": [[0.2, 0.24]],
+            }
+
+    monkeypatch.setattr("atocore.retrieval.retriever.get_vector_store", lambda: FakeStore())
+    monkeypatch.setattr("atocore.retrieval.retriever.embed_query", lambda query: [0.0, 0.1])
+    monkeypatch.setattr(
+        "atocore.retrieval.retriever._existing_chunk_ids",
+        lambda chunk_ids: set(chunk_ids),
+    )
+    monkeypatch.setattr(
+        "atocore.retrieval.retriever.get_registered_project",
+        lambda project_name: type(
+            "Project",
+            (),
+            {
+                "project_id": "p05-interferometer",
+                "aliases": ("p05", "interferometer"),
+                "ingest_roots": (),
+            },
+        )(),
+    )
+
+    results = retrieve(
+        "interferometer error budget vendor constraints",
+        top_k=2,
+        project_hint="p05-interferometer",
+    )
+
+    assert len(results) == 2
+    assert results[0].chunk_id == "chunk-requirements"
+    assert results[0].score > results[1].score