p1-6: rename StoreError::Sqlx -> Sqlite, drop dead assets_root helper

M1: `Sqlx` is a misleading leftover — this crate uses `rusqlite`, not
sqlx. Rename the variant (and the doc reference to it) to `Sqlite`. No
external pattern matches; the variant is reached only via `#[from]`.

M11: `assets_root` was an `#[allow(dead_code)]` helper introduced for a
test that never landed. Delete it so the dead-code allow goes with it.

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

crates/kb-store-sqlite/src/error.rs

@@ -2,7 +2,7 @@
 //!
 //! Boundary code (`kb-app`, `kb-cli`) flattens these into `anyhow::Error`,
 //! so the trait impls return `anyhow::Result` directly. Internally we
-//! still distinguish `Conflict` (e.g. checksum mismatch) from `Sqlx` /
+//! still distinguish `Conflict` (e.g. checksum mismatch) from `Sqlite` /
 //! `Migration` so callers that downcast can route refusal-style flows.
 
 use thiserror::Error;
@@ -10,7 +10,7 @@ use thiserror::Error;
 #[derive(Debug, Error)]
 pub enum StoreError {
     #[error("sqlite error: {0}")]
-    Sqlx(#[from] rusqlite::Error),
+    Sqlite(#[from] rusqlite::Error),
 
     #[error("migration error: {0}")]
     Migration(String),

crates/kb-store-sqlite/src/store.rs

@@ -397,10 +397,3 @@ fn expand_data_dir(raw: &str) -> PathBuf {
 fn dirs_home_fallback() -> Option<PathBuf> {
     None
 }
-
-/// Returns the root of the assets shard tree (`data_dir/assets/`). Used
-/// by tests; kept crate-private otherwise.
-#[allow(dead_code)]
-pub(crate) fn assets_root(data_dir: &Path) -> PathBuf {
-    data_dir.join(ASSETS_SUBDIR)
-}