fix(storage): drop ineffectual keyHash initial assignment

ineffassign caught `keyHash := base` in FileIDFromPath being
unconditionally overwritten by the suffix-strip switch (or
overridden by the default branch's early return). Switching to
`var keyHash string` makes the intent explicit: the switch assigns
in every path that doesn't return.

Pure cleanup. Pre-pushed through the same golangci-lint v2.0.2
binary CI uses; full repo now reports '0 issues'.

Author: 2nd1st

internal/storage/canonical.go

@@ -161,7 +161,7 @@ func FileIDFromPath(dataDir, path string) (FileID, error) {
 		return FileID{}, fmt.Errorf("FileIDFromPath: date segment %q does not match YYYY-MM-DD", date)
 	}
 	// Strip the suffix. Order matters: .jsonl.gz first, then .jsonl.
-	keyHash := base
+	var keyHash string
 	switch {
 	case len(base) > len(".jsonl.gz") && base[len(base)-len(".jsonl.gz"):] == ".jsonl.gz":
 		keyHash = base[:len(base)-len(".jsonl.gz")]