feat: query-relevance ordering for memory selection

get_memories_for_context now accepts an optional query string.
When provided, candidate memories are reranked by lexical overlap
with the query (stemmed token intersection, ties broken by
confidence) before the budget walk. Without a query the order is
unchanged — effectively "by confidence desc" as before — so
non-builder callers see no behaviour change.

The fetch limit is raised from 10 to 30 so there's a real pool to
rerank. Token overlap reuses _normalize/_tokenize from
reinforcement.py so ranking and reinforcement matching share the
same notion of distinctive terms.

build_context passes the user_prompt through to both the identity/
preference and project-memory calls. The retrieval harness
regression the fix is targeting:

- p05-vendor-signal FAIL @ 1161645: "Zygo" missing from the pack
  even though an active vendor memory contained it. Root cause:
  higher-confidence p05 memories filled the 25% budget slice
  before the vendor memory ever got a chance. Query-aware ordering
  puts the vendor memory first when the query is about vendors.

New regression test test_project_memories_query_relevance_ordering
locks the behaviour in with two p05 memories and a tight budget.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_context_builder.py

@@ -308,3 +308,41 @@ def test_project_memories_absent_without_project_hint(tmp_data_dir, sample_markd
     pack = build_context("tell me something", budget=3000)
     assert "--- Project Memories ---" not in pack.formatted_context
     assert pack.project_memory_chars == 0
+
+
+def test_project_memories_query_relevance_ordering(tmp_data_dir, sample_markdown):
+    """When the budget only fits one memory, query-relevance ordering
+    should pick the one the query is actually about — even if another
+    memory has higher confidence.
+
+    Regression for the 2026-04-11 p05-vendor-signal harness failure:
+    memory selection was fixed-order by confidence, so a lower-ranked
+    vendor memory got starved out of the budget when a query was
+    specifically about vendors.
+    """
+    from atocore.memory.service import create_memory
+
+    init_db()
+    init_project_state_schema()
+    ingest_file(sample_markdown)
+
+    create_memory(
+        memory_type="project",
+        content="the folded-beam interferometer uses a CGH stage and fold mirror",
+        project="p05-interferometer",
+        confidence=0.97,
+    )
+    create_memory(
+        memory_type="knowledge",
+        content="vendor signal: Zygo Verifire SV is the strongest value path for the interferometer",
+        project="p05-interferometer",
+        confidence=0.85,
+    )
+
+    pack = build_context(
+        "what is the current vendor signal for the interferometer",
+        project_hint="p05-interferometer",
+        budget=1200,  # tight enough that only one project memory fits
+    )
+    assert "Zygo Verifire SV" in pack.formatted_context
+    assert pack.project_memory_chars > 0