fix(migrate): write a real clawhdf5-agent store

clawhdf5-migrate wrote a layout of its own (/chunks, /sessions,
/entities, /relations, root attributes, no /meta or schema_version) that
HDF5Memory::open rejects, so a "migrated" SQLite database could not be
used as agent memory — contrary to the README.

It now writes through the agent's own API (HDF5Memory::create/open,
save_batch, the session cache and the knowledge graph), so there is no
second copy of the schema:

- sessions and entities/relations carry over; deleted rows become
  deleted records (or are left out with --skip-deleted);
- embeddings follow the library default (float16), --f32 opts out and
  --float16 is a hidden no-op, as in clawhdf5-cli; the `half`-based
  conversion is gone;
- every source row is checked before the output is created: a wrong
  embedding length, an empty embedding, a dimension that differs from
  an existing store's, or a float16 value beyond +-65504 is an error
  naming the chunk id, and an existing store is left untouched;
- --incremental opens the existing store, adds only rows it does not
  hold (matched by content) and follows the source's deleted flags;
- a source with no memory rows needs --embedding-dim;
- validation reads the result back with HDF5Memory::open_read_only,
  compares every field (embeddings bit for bit, round_to_f16 of the
  source for float16) and checks a migrated record is found by search.

clawhdf5-agent gains HDF5Memory::sessions()/sessions_mut(),
HDF5Memory::delete_batch (one save, all-or-nothing, no auto-compact),
SessionCache::add_at, and re-exports SessionCache/SessionEntry.

The old layout's per-dataset SHA-256 provenance attributes have no place
in the agent schema and are gone. An adversarial review found two
blockers (silent truncation of long embeddings; an --incremental
dimension check that could never fire) and four majors (a failed run
wiping the existing store, dim-0 stores, deleted-flag drift); all are
fixed with regression tests. 42 migrate tests, incl. h5py opening a
migrated store.

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

crates/clawhdf5-agent/src/session.rs

@@ -33,7 +33,7 @@ impl SessionCache {
         self.entries.is_empty()
     }
 
-    /// Add a new session with its summary.
+    /// Add a new session with its summary, timestamped now.
     pub fn add(
         &mut self,
         id: &str,
@@ -47,6 +47,21 @@ impl SessionCache {
             .unwrap_or_default()
             .as_secs_f64()
             * 1_000_000.0; // microseconds
+        self.add_at(id, start_idx, end_idx, channel, summary, ts);
+    }
+
+    /// Add a session with an explicit timestamp (Unix **microseconds**, the
+    /// unit [`SessionEntry::ts`] uses) — for importers carrying sessions over
+    /// from another store, whose original time should be kept.
+    pub fn add_at(
+        &mut self,
+        id: &str,
+        start_idx: usize,
+        end_idx: usize,
+        channel: &str,
+        summary: &str,
+        ts: f64,
+    ) {
         self.entries.push(SessionEntry {
             id: id.to_string(),
             start_idx: start_idx as u64,