feat(agent): expose the HNSW parameters in MemoryConfig

Graph degree and the build- and query-time candidate list sizes were
constants, so a deployment had no way to trade recall against memory or
query speed. They are now `MemoryConfig::hnsw_m`,
`hnsw_ef_construction` and `hnsw_ef_search`, persisted with the store
and defaulting to exactly the previous behaviour (16, 64, and a query
list that scales with `k`).

Two things the straightforward version would have got wrong:

`clawhdf5-ann` asserts a graph degree of at least 2, so a configured 0 —
from a file, or from a caller reading 0 as "use the default" — aborted
the process inside the index builder. The store clamps instead, and a
test covers it: removing the clamp makes that test panic rather than
fail.

`ef_search` and the candidate pool handed to score fusion were the same
number. Tying the pool to the new setting would mean lowering `ef` for
speed also narrows what fusion sees, quietly degrading hybrid results
through a knob that looks like it only costs time. They are now
independent.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

crates/clawhdf5-agent/src/schema.rs

@@ -108,6 +108,15 @@ pub fn build_hdf5_file_with_meta(
         "quantized_index",
         AttrValue::I64(config.quantized_index.into()),
     );
+    meta.set_attr("hnsw_m", AttrValue::I64(config.hnsw_m as i64));
+    meta.set_attr(
+        "hnsw_ef_construction",
+        AttrValue::I64(config.hnsw_ef_construction as i64),
+    );
+    meta.set_attr(
+        "hnsw_ef_search",
+        AttrValue::I64(config.hnsw_ef_search as i64),
+    );
     meta.set_attr(
         "edgehdf5_version",
         AttrValue::String(ZEROCLAW_VERSION.into()),
@@ -489,6 +498,15 @@ pub fn validate_and_load(
             .and_then(|v| usize::try_from(v).ok())
             .unwrap_or(500),
         quantized_index: optional_bool_attr(&attrs, "quantized_index", false),
+        hnsw_m: optional_i64_attr(&attrs, "hnsw_m")
+            .and_then(|v| usize::try_from(v).ok())
+            .unwrap_or(16),
+        hnsw_ef_construction: optional_i64_attr(&attrs, "hnsw_ef_construction")
+            .and_then(|v| usize::try_from(v).ok())
+            .unwrap_or(64),
+        hnsw_ef_search: optional_i64_attr(&attrs, "hnsw_ef_search")
+            .and_then(|v| usize::try_from(v).ok())
+            .unwrap_or(0),
     };
 
     // Load /memory group