feat(store-sqlite): add truncate_embedding_records helper

Wipes every row from embedding_records and returns the deleted row
count. Used by the upcoming `kebab reset --vector-only` to keep SQLite
consistent after the on-disk Lance store is removed.

Plan deviation from the original spec (task 1):
- Original test plan opened SqliteStore with a raw path; the actual
  signature is `SqliteStore::open(&Config)`, so the integration test
  builds a Config with `storage.data_dir` pointed at a tempdir.
- Original return type was Result<()>; bumped to Result<u64> so the
  caller (kebab-app::reset) can surface the truncated count in the
  reset_report.v1 wire payload without a separate COUNT query.

p9-fb-06 task 1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-store-sqlite/src/embeddings.rs

@@ -107,6 +107,22 @@ impl SqliteStore {
     /// WHERE embedding_id IN (?, ?, …)`) inside one transaction —
     /// avoids the per-row `execute()` round-trip the previous
     /// implementation paid.
+    /// Wipe every row from `embedding_records`, returning the count of
+    /// rows that were removed. Called by `kebab reset --vector-only` so
+    /// SQLite cannot point at a Lance row that the reset just removed
+    /// off-disk.
+    ///
+    /// The function does NOT cascade to `chunks` or `documents` — those
+    /// are kept so the next `kebab ingest` re-embeds the existing chunk
+    /// set without re-parsing.
+    pub fn truncate_embedding_records(&self) -> Result<u64> {
+        let conn = self.lock_conn();
+        let n = conn
+            .execute("DELETE FROM embedding_records", [])
+            .context("DELETE FROM embedding_records")?;
+        Ok(n as u64)
+    }
+
     pub fn mark_embedding_records_committed(
         &self,
         embedding_ids: &[String],