feat(agent): new stores default to float16 embeddings

MemoryConfig::float16 now defaults to true for new stores, on
measurement: on the full LongMemEval haystack with real MiniLM
embeddings every retrieval metric matched f32 (previous commit), and at
100K the file is 48% smaller with faster checkpoints and opens.

Existing stores are unaffected: every agent store has recorded
`float16 = false` in /meta and keeps it. A test opens the v2.5.0
fixture, saves and checkpoints, and checks the embeddings are still f32
with the old rows bit-identical; another checks a new store is float16.

CLI: `create --f32` opts out; like `--f32-index` it only ever switches
the default off. `--float16` is still accepted and now a no-op.
Values beyond +-65504 are refused, so f32 remains the choice for
unnormalised vectors — the upgrade note says so.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-agent/src/lib.rs

@@ -136,6 +136,13 @@ pub struct MemoryConfig {
     /// so search results are the same before and after a reopen. Values must
     /// lie within ±65504; a save outside that is `MemoryError::InvalidEntry`.
     /// Fixed when the store is created (persisted in `/meta`).
+    ///
+    /// **On by default for new stores**: on the full LongMemEval haystack with
+    /// real MiniLM embeddings every retrieval metric matched `f32`, and at
+    /// 100K records the file is 48% smaller (`BENCHMARKS.md`). Existing
+    /// stores keep the setting they were created with. Set it to `false` for
+    /// full-precision embeddings, e.g. for unnormalised vectors that may
+    /// exceed the half-precision range.
     pub float16: bool,
     pub compression: bool,
     pub compression_level: u32,
@@ -191,7 +198,7 @@ impl MemoryConfig {
             embedding_dim,
             chunk_size: 512,
             overlap: 50,
-            float16: false,
+            float16: true,
             compression: false,
             compression_level: 0,
             compact_threshold: 0.3,