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_context_builder.py

@@ -41,6 +41,23 @@ def test_context_with_project_hint(tmp_data_dir, sample_markdown):
     assert pack.total_chars > 0
 
 
+def test_context_builder_passes_project_hint_to_retrieval(monkeypatch):
+    init_db()
+    init_project_state_schema()
+
+    calls = []
+
+    def fake_retrieve(query, top_k=None, filter_tags=None, project_hint=None):
+        calls.append((query, project_hint))
+        return []
+
+    monkeypatch.setattr("atocore.context.builder.retrieve", fake_retrieve)
+
+    build_context("architecture", project_hint="p05-interferometer", budget=300)
+
+    assert calls == [("architecture", "p05-interferometer")]
+
+
 def test_last_context_pack_stored(tmp_data_dir, sample_markdown):
     """Test that last context pack is stored for debug."""
     init_db()